I hear you on the "it must be aliens!" argument. Really, I do. And until at least the concept of the "Alcubierre drive" came out, one could simply claim "there's no possible way for any being to physically come here from any galactic-level distance because of the amount of time it would take at sub-light speeds" and that would be the end of the story. (Even then, my response was usually "no way
that we can think of"...)
I am well aware of all of your other arguments, as I was a Psych major who specialized in perception.
I'm of the government's stance on this currently: I think it's a real phenomenon, I trust most qualified eyewitnesses especially if it is in concordance with radar data, and I refuse to concretely claim anything further. And it is a mystery.
But I AM free to speculate.
I could claim something like "assuming there IS some alien intelligence controlling these hyper-performing craft... if another intelligence was advanced enough to even make it here in the first place, ostensibly they also have fairly full control over all elements of the interactions with us, which includes not showing themselves outside their craft", but this is also the Conspiracy Fallacy. Two thoughts on that though: The Conspiracy Fallacy does not disprove conspiracies (which is why I am openly stating this is speculative). And secondly, this also means that (if this claim is at all true), they are likely voluntarily showing themselves at some "pace" as to not overly upset us. Why that? Well, perhaps you've heard of systematic desensitization (https://en.wikipedia.org/wiki/Systematic_desensitization) as a treatment for phobias? That might be going on here. And it's apparently working, since nothing crazy's happened since the USG openly admitted this is real. This is a far cry from the Orson Welles' "War of the Worlds" broadcast days.
So, no. I can't say it's aliens, at least rationally. But I do speculate that. ;) And honestly, I think it's fun to think that not only might we be not alone (wouldn't that be amazing??) but that we're kind of in a galactic kindergarten. ;)