If aliens were visiting Earth they would have left something behind: A cigarette butt. A soft drink can. A gum wrapper. But we have -- nothing.
Yeah but, you are thinking like a 20th century man, speculating on a life which might be a million years ahead of us in evolution and technology. You might as well show a fighter jet to a neolithic backwoodsman in Borneo. And FWIW, I say that as a RETIRED scientist (chiefly an engineer, I've worked in a scientific capacity as well), because, you see, I've seen a few things I cannot explain, both naked eye and telescopically, and well, back in the late '60s, I saw something which simply violated all known laws of physics even for today.
I can rationalize the other things I saw as maybe weather balloons, that sort of thing, but that ONE TIME, I know what I saw, and as a person schooled in math, science and physics who knows other people still working in R&D, either we have technology WAY WAY ahead of anything openly known to the public (a remote possibility), or, there is something very strange going on out there that if I were a government of this Earth, I'd be very concerned about.
And if the former, then something every person living ought to be very concerned about!
Either way, it ain't good, and to try to rationalize or dismiss it all on the expectation that an interstellar traveler must obviously operate like you to be taken seriously, is frankly, just a cop out to avoid dealing with facts which neither you nor I can explain.
