One possibility is that if we are talking about biological life, it might not be possible for life from planet A to digest matter from planet B. After all terr

One possibility is that if we are talking about biological life, it might not be possible for life from planet A to digest matter from planet B. After all terrestrial life has special enzymes and processes to deal with other terrestrial life. In that case if you wanted to colonize another planet you’d have to look for an empty one, or you’d have to erase the whole native ecosystem and start over. Looking for an empty planet might be a lot easier, therefore expansionist civilizations might be specifically avoiding inhabited planets. Maybe?