This is a brilliant worldbuilding choice, because it makes the galaxy feel as big and diverse as it ought to feel, and at the same time it sets things up for more imports. Galaxies are huge. The space covered by the UF stories is explicitly not the whole galaxy, and yet we know that there are thousands, maybe millions, of star systems within the explored volume that we haven't heard about yet. It is easy to believe that any one of these might be home to the next civilization, original or imported, that will appear on stage when the narrative requires it. It's happened many times before.