#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.