Deme

Climate change is just one of six planetary boundaries that we’ve crossed, out of a total of nine. The choice of rocket fuel is largely inconsequential compared to the effects of maintaining the industrial capacity necessary for such endeavours.

#1: I doubt there would ever be a situation where those same resources wouldn’t be better used to make things slightly less unbearable on the home world. In our case, even if we covered the world in poison and had an endless nuclear winter, Mars would still look like the worse planet to live on. It’s doubtful whether or not a better one exists within any “practical” distance. If the aliens happened to have a lucky spawn in a star system with multiple habitable planets, good for them. They have another chance to figure things out. But interstellar flight (not to mention colonization) is still vastly more difficult.

#2: Exploiting the resources of the solar system is orders and orders of magnitude simpler than establishing self-sufficient colonies in uninhabitable space or planets. The show For All Mankind threw out most of any believability it had a while ago, but even there the entire fourth season revolved around the subject of how even a single asteroid full of rare earth metals would sate our hunger for such a long time as to effectively kill any initiatives to expand in space.

Space exploration necessitates a technological industrial civilization. So they/we would somehow have to figure out how to first do #2 (so as to not die), while still maintaining the industrial capacity to spread out into space. That sounds like an even more improbable subset of the already improbable scenario #2.

That distance exists not only in space, but most likely time as well. Extrapolating from our singular data point, it would seem that the lifespan of a technological civilization is quite short. The odds of two of those being around at the right times for even one of them to detect the passing emission shell of the other is diminishingly small.

My thinking is that a technological species either goes into ecological overshoot so badly that it kills itself (or at least its capacity to conquer space) ((this is what we’re doing currently)), or then it learns to live harmoniously as a functioning part of the wider planetary system, and thus has no need to spread into space.