And if interstellar travel is possible it's more than likely there are civilizations out there that have cracked it.
And if that's the case what's so implausible about aforesaid civilizations wanting to pay us a visit? If we had the technology (and I believe we will eventually) we'd be raring to go.
I'm not saying UFOs are for real (though I think they are) just that I see no very good reason for dismissing them out of hand- apart, that is, for an understandable reluctance to surrender our current (rather cosy) view of the universe.