Here's a chain of reasoning which might explain why aliens haven't come knocking at our doors. First, just assume there are a lot of them, they maintain their civilizations by migrating, they communicate with other inhabited worlds, they have star travel, and from time to time, one solar system just runs flat out of resources and has to be abandoned. They move their civilization somewhere else, as they are category A2 aliens, who value their civilization and want to preserve it.
Second, colonization doesn't mean everybody on a planet packs up and gets on board a stellar transport and heads over to a new planet. That's way too costly. Even sending a colonization team might be too expensive, and also, unnecessary. We need to ask what their goals are, and the answer is, by assumption of what category alien we are talking about, preservation of their civilization. This does not mean preservation of their population by moving them. Scarcity doesn't happen fast, but very slowly. There is no need for evacuation. Just for a colonization robot ship to go and bring everything necessary to re-construct their civilization there.
Think of the cost differential. Robotics doesn't require life support, which for a hundred to thousand year voyage, is immense. Asymptotic robotics, coupled with asymptotic genetics, would make setting up a new planet on a suitable world, a sweet spot world, child's play.
Don't imagine the negatives are like a famine on Earth. Nobody is running out of anything on the alien planet. They see their resources are running dry, do the starship thing, and gradually reduce their population over the centuries. They are busy substituting resources that are still available for resources which are growing to costly to obtain. Meanwhile they are waiting for communication to come back from the new world, so everybody will have a happy feeling about fulfilling their meme of civilization preservation.
They are all brilliant, so they see scarcity coming, and long before any crisis might occur, make plans to deal with it. Perhaps they do two or three starships, just to cover their bets. Reliability is will understood after asymptotic technology, and recreating life in a lander on the new planet will simply follow well-understood procedures, done on the home planet millions of times. There is one question however.
Would they need to send a probe to the potential new planets to do some detailed measurements before sending out the colony ship? The probe might be one tenth the size of the colony ship, and if there was a lot of uncertainty, some alien leader might suggest sending out ten or twenty, just to pick the best worlds. But is it necessary? Could an alien civilization use very large observatories in their home solar system to figure out everything they needed to know? Are probes superfluous?
To try and get some insight into that, think about what they would want to know. Just assume for the discussion they live on the solo planet that spawned them, or something similar to it. It is a tectonically quiet planet, with breathable air, life forms related to them everywhere in the natural areas or maybe life forms related to them by DNA type that they have created and used to replace the natural ones, not too much UV or radiation, full of resources, and probably a few more. This is a sweet spot world. It is just waiting for intelligent life to come to it. Can you tell one from a distance?
With a mind-boggling observatory in their solar system, out far from the sun for quiescence, able to sit and look for a century at potential home planets, these things can be figured out. Imaging the planet would be possible with kilometer-scale telescopes, and perhaps they could be built large enough to see if there were polar ice caps or land masses at middle latitudes. The atmosphere would be measured very accurately, and used to model the planet. The consistency of the model with all the known measurements would provide a great deal of confidence in what was on these planets. They could not rule out tectonics, but volcanoes put signatures into the atmosphere, and if you are monitoring the atmosphere for a century, you at least know how much volcanism went on during that time.
UV doesn't need the huge observatory to measure it, that is an easy one, knowing the sun and the atmosphere of the planet. What about radiation?
Let's not forget that the data from the observatory looking at these candidate planets is not the only source of information they have. They have generic information. They know how planets form, they know how to estimate what was in the premordial dust cloud that condensed into the planets. They of course know the lifetimes of radioactive elements. So they can comfortably predict the radiation.
Let's also not forget they have passed the genetic grand transition. They are not worried about some virus being present on the planet that would infect all the colonized life, and kill it off. Virus checks would be routine on their planet, and furthermore, their master computer network would be able to generate all possible types of viruses, and determine plans for detecting and eliminating them.. The same goes for microbes. What about hungry carnivores? Not a problem for defense.
They wouldn't even send a probe, as the remote observatory would be able to tell them whatever they wanted. The only ship to come would be the colonizing one.
This leaves one obvious problem. What about another alien civilization on the planet, not so far along in their development, but there and maybe in the hunting stage or the agricultural stage or the early industrial stage, or maybe later. Their starship is going to take centuries to get there, and what is such a civilization going to develop into within that travel time? Will they see the lander and set about to destroy it? This could turn a sweet spot world into a disaster. Could the aliens contemplating colonization figure out how to eliminate the other alien species without damaging the planet in some way. How hard would it be to do that? Doesn't that depend on how many of them there are and what technology they have accomplished? Even if they could measure how many of the new type of aliens were on the planet, how many would there be after a few centuries of travel time?
What exactly might be their strategy for taking over this planet, or rather taking it away from the homebodies? Robot exterminators – how to make enough of them? Poisons – how do you get it to all the aliens? Some plague – probably some would be immune. Maybe the best bet is to pick a planet with no civilization there, or where a civilization had destroyed itself.
So, if there have been a civilization nearby us, with the right timing so they were at the cusp of scarcity, and they looked at Earth in the last twenty thousand years with a gigantic observatory, perhaps they said, 'No, thanks, keep the planet, we'll go to another one.' If getting rid of aliens, especially ones with a nasty tendency to want to hang onto their only planet, is the hardest problem a colonist might face, this explains why nobody has come here. It's us.
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