1) known US aircraft activity either simply misinterpreted or kept secret from the public or even most of the military apparatus, sometimes strongly denied either as a way to disseminate vague information to adversaries or misinformation
2) foreign or personal aircraft either known or unknown to the US.
3) rare weather phenomena, hallucinations, optical phenomena, or other miscellaneous things which might be rare but with mundane explanations
The thing to notice is cameras have become much more prevalent and much higher resolution, but "UFOs" in pictures haven't. The only things that aren't obviously something understandable are still fuzzy dots on screens despite screens and cameras getting enormously better.
Just look at this https://media.gvwire.com/wp-content/uploads/2021/01/28154540... and tell me that if you saw that you wouldn't think it was a spaceship. It was kept super secret for a quite a long time and surely was the source of plenty of UFO reports.
I'm not convinced or even particularly suspicious that anything that "isn't what it seems" is anything more than mundane.