Because on the face of it, they certainly seem to.
Planet #1: Mondas, aka Blowy-uppy-world.
Mondas, I don't like Mondas, I'm going to burn the whole planet down. Though, ironically, that's what they try to do to Telos, and only blow up Mondas by mistake.
In "The Tenth Planet", astronomers from Earth SUDDENLY spot a ruddy great planet barrelling down on us. Now, even though this was set in the DISTANT FUTURE of, er, 1986, and therefore before Mr Lembit had got all shouty about asteroids, a planet the size of Earth (which, by definition, Earth's TWIN planet would be) ought to be visible for millions of kilometres. You know, like Mars and Venus are. And yet this comes as a total surprise to people who are running a SPACE PROGRAMME and therefore jolly well ought to know. Flying a rocket isn't just Mirror, Signal, Manoeuvre, you know.
And yet somehow, Mondas just "appears" from nowhere.
Planet #2: Telos, aka Freezer Empire (catchy advert: "Mum's Gone to Telos")
Although Dr Who returns there in the DISTANT, er, PRESENT DAY of 1985 for the "Attack of the Cybermen", the Cybermen's home from home – or rather tomb from tomb – is first visited in "The Mummy". Er, what? No, you see, "The Tomb of the Cybermen" is basically a retelling of the Mummy legend, with Telos as the Egyptian desert and Cyber-control as the lost pyramid. The hieroglyphs, sorry "logic-symbols", are the big give away. Oh, and the lumbering Undead rising from the tomb to exact their curse on whomsoever should disturb their eternal slumber. Anyway, this, in and of itself, relies on Telos being a "lost planet", somehow difficult to find. Invisible even.
But then to compound that, there's the evidence that Telos is actually IN our own Solar System. To reprise the whole Planet 14 debate: all of the mighty Trout, Mr Dr Pat's, adventures with the Cybermen are set in the Solar System and in the 21st Century (or late 20th), and all of them make it pretty clear that the Cybermen do NOT have interstellar technology and desperately need to conquer the Earth as their only hope of survival. Well, they all do so long as you accept the allowances you have to make if you're to make ANY sense of that one written by Mr David "I wouldn't know the difference between a Galaxy and a Finger of Fudge" Whitaker. Given that going cryo is their very-last-throw-of-the-dice option, that would suggest that the oh-so-difficult-to-find Telos is right here in the Solar System. Which would have to make it even more difficult to detect.
So ARE they invisible?
Well, there IS another possible explanation, although it's a bit "Star Trek".
The alternative is that there is some kind of WORMHOLE near to where the Earth orbits the Sun. And Mondas fell through it.
This isn't QUITE as unlikely as it sounds, at least not in the Doctor Woo universe. There's at least one occasion when a pretty ordinary Earth spaceship (i.e. one of the ones that THAT stock-footage from NASA can double for) manages to accidentally get lost in another star system. Admittedly that's "The Android Invasion" aka "The One with the Spoiler in the Title" (or more precisely "The One with the Spoiler in the Title that doesn't have Daleks in it"). Nor is it unknown for planets to apparently wander in and out of the Solar System – Voga in "Revenge of the Cybermen", Vulcan in "Power of the Daleks", arguably the Moon in "…and the Silurians" – so there's a least a chance that trans-galactic billiards is on the cards.
And of course, the whole of Torchwood is based on the idea that there is this space-time rift to anywhere that just happens to have a back door in Cardiff.
Actually, the Torchwood Rift (or GIANT ANTI-PLOTHOLE as it is called) ought to be a tiny wee crack in the Universe (what with the Gelth specifically NOT able to come through in numbers) rather than the bleedin' enormous demon-disgorging dimensional doorway that it is used as whenever the plot calls for it to be the Hellmouth this week, but N E Waaaaaay.
It's been put forwards by others that the REASON the Earth keeps getting invaded in the 1970s and/or 1980s (your UNIT-dating mileage may vary) is because there is some STRATEGIC value to the planet beyond the prettiness of its oceans and the nice view towards Alpha Centauri. A convenient hyper-space by-pass to the Oseidon system (or wherever) would certainly be that.
Mondas popping back thorough the rift would satisfy the "suddenly appearing from nowhere" criterion that "The Tenth Planet" sets up; and if Telos is at the other end of the rift, especially if the other end isn't fixed (yes, just like that one with the Ferengi) then that would make it the "lost city/planet/tomb" for which Professor Parry's team had to scour the universe. (Even though they seem ill-equipped to scour a BATHTUB!)
But that's not really "drifting to the edge of space" is it. It's rather more "blimey, where did the sun go?" and "where's parallel Professor Richard to tell us that WE'VE moved and not all these new planets" to be honest.
No, the wormhole explanation won't do.
Instead, let's pretend that the Cybermen behave logically like they claim they do (but DON'T!) and look at this that way.
This whole problem starts with Mondas becoming increasingly uninhabitable as it drifts away from the Sun. The Mondassians have got time and technology on their side, but otherwise they're more STUFFED than I am.
We know that their EVENTUAL solution was to turn themselves into friends of Dorothy. (OK, so the straw cybermen and the cowardly cat-monster Mondassians were even less successful…) But what else might they have considered?
Well, large scale INSULATION of the planet might be a good idea. Your main needs are to keep the atmosphere in and to keep it warm: what you want is some sort of dome over much if not all of the planet, an artificial sky that is reflective on the inside to retain as much heat as possible and is totally black on the outside to absorb any warming sunlight that manages to fall upon it.
Entirely as a BY-PRODUCT of that technology, you've built a great big NINJA COSTUME for your planet, rendering it difficult if not impossible to see.
Of course, returning to the environs of the Earth, such a shroud would result in the REVERSE problem, runaway global warming. And we already know that Mondas suffers from overheating. The Cybermen would need to pretty quickly disassemble their planetary blanket… hence apparently materialising out of nowhere!
That's Mondas explained… what about Telos?
Well, ask yourself about the native Telosians, the Cryons who we meet in "Attack of the Cybermen", a species so evolved for sub-zero conditions that they VAPORISE at room temperature. And yet, humans appear able to wander about on the surface of their home planet without instantly turning to popsicles. So, what, the Cryons invented the freezer BEFORE they evolved? It seems a little unlikely. Obviously, something has happened to CHANGE their planet – and what more obvious a change than the invasion of the Cybermen.
Actually, even this doesn't really make total sense. We're supposed to believe that the Cybermen invaded Telos to take advantage of the Tombs… so the giant fridges must have been there before them… so they can't have been built BECAUSE of the Cybermen. But let's take a little CREATIVE licence. Say the Cybermen invade Telos just because. After all, it's a planet and their own world, Mondas, is currently on an apparently one-way trip to deep space doom. But the Telosian weather is routinely what we call "a bit nippy" or even "quite brisk". And yet fortunately the Cybermen just HAPPEN to HAVE the technology to hand for exactly these circumstances: they adapt same techniques they used on Mondas and set about adjusting the Telosian climate to be a bit more to their liking. And, as the temperature rises, the Cryons are forced to build refrigerated cities just to survive the Global Warming.
In this sense, Telos is twin-of-terraformed into a planet with Earth-like surface conditions (and quarries) encased in a black light-absorbing suit making it vanish. The only way to detect it would be by calculating the effect that its gravity has on the surrounding system… enter the Brotherhood of Logicians with their "mass intelligence" able to put in the thousands of man hours of calculations needed to locate the lost planet.
On the other fluffy foot, ASTROLOGY rather than astronomy would be more in keeping with the whole "ancient curse" vibes. And the Cybermen ARE the astrologers of the Doctor Who Universe.
After all, their plans begin with the movement of the planets ("The Tenth Planet"), before progressing to the Moon ("The Moonbase"), a nova in another galaxy(!)… oh all right, meteorites ("The Wheel in Space"), a planet of gold ("Revenge of the Cybermen") and finally turning into a falling star themselves ("Earthshock"), even if they wind up chasing a "magic comet" ("Silly Nemesis").
Of course an INVISIBLE planet would turn an astrological chart to gibberish… if it wasn't gibberish ALREADY! Maybe that is why all of the Cybermen's schemes go wrong.
"Your Stars for Today. Telos: a tall/short dark/fair young/old stranger will be important to your plans. Maybe don't bother going out."