Why would Earth be long-term unsuitable for an advanced alien species that's already colonized it?
I'm writing a story where aliens are using humanity like a botnet to do hyper complex calculations - each infested human has a program running in their subconscious brain that does a small part of the overall computation. They're doing this in order to find a habitable planet, since they've more or less depleted their homeworld. Their system of FTL travel requires nearly perfect accuracy in terms of coordinates, so they're running simulations to be absolutely certain the next planet they reach is habitable before using up expensive resources to get there. At the moment, most of the aliens live on a mothership orbiting Earth, while a few are disguised as humans and some have been on Earth for 20 years.
That last point is where my problem lies - if some aliens are on Earth, why don't they all settle there permanently? Clearly they're capable of living on Earth for extended periods of time, but for the story I need them to have ruled out Earth as their home the moment they arrived. Also you would assume they ran some simulations prior to coming here, since their FTL is so expensive. What about Earth or our solar system would be hard to predict and makes living here untenable, without being impossible in the short term?
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1 answer
"It's the smell!"
-- Smith
I think the easiest answer is that there is something in Earth's biology that disagrees with them... something that can be "managed" with continuous medical intervention, but would be really annoying for them to try to settle permanently without completely terraforming the planet. Imagine, for example, that 99% of the aliens experience the equivalent of severe allergies when in contact with our biome. Sure, maybe they can damp it down to a manageable level with the equivalent of antihistamines, but could you imagine living for generations under those conditions? Well, neither can they.
So why don't they just terraform the planet? Well, aside from the possibility that they'd rather not exterminate us (as mentioned in other answers), maybe they don't have the technology, or even if they could, maybe it would take thousands or even millions of years for the planet to become tolerable again after sterilizing it (and this assumes that they're sure they could fix whatever caused the problem in the first place) before the planet would be worth living on. It's quicker and easier to just roll the dice again.
It seems plausible that there could be some subtle biological thing that their models didn't predict that would be suitable. I also think suffering from near-constant hayfever would make for good story-telling.
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