Except we should have noticed a generation ship. Particularly if we're assuming aliens not much more advanced than we are. It would be big and it would give off a lot of heat and it would have to be in orbit. I think you're underestimating the technological complexity of building what is essentially a sealed, self-sustaining biosphere capable of running for tens of thousands if not millions of years. Maybe on a napkin we're anywhere near able to do something like that.

Plus this goes against the common narrative that these craft display propulsion capabilities that appear to defy physics, which (if we're assuming aliens) implies anti-gravity at a minimum. No one is claiming to encounter UAPs or UFOs with wings and conventional rockets. Again, that's something we only kind of know how to do on a napkin, and even then only if we fudge the numbers.

At the very least, I would want to assume any civilization capable of interstellar travel to be at least 1000 years ahead of us, technologically. Even with generation ships and no space magic. It's just a big stupid round number but I intend it to mean generally as far ahead of us now, in terms of scientific knowledge and technological progress, and assuming a few paradigm shifts ahead, as we are ahead of the Middle Ages.