The terminology “Unidentified Flying Objects” is itself pretty misleading. It’s very clear that many supposed UFOs that have been successfully identified through the years have not been Flying, and many of them have not been Objects.
As noted above, it’s not as if there’s one single explanation for all of these. A range of phenomena have been pointed to as “OFOs”, and a number of different things lie behind the ones that have been identified.
Some have been re-entering spacecraft, or launch tests, or even meteors. These aren’t flying objects – most of them are falling objects.
Some have been identified as aircraft performing missions or tests, often at night. These were definitely flying, but , although not identified by the spotters, they weren’t unidentified by everyone.
Several UFOs have been very plausibly identified as astronomical objects – bright stars, or planets, or even the moon, especially when seen through haze.
As Donald Menzel (one-time director of the Harvard Smithsonian Observatory, editor of :Fundamental Formulas in Physics", and author of three books debunking UFOs) once pointed out, many "UFO"s aren’t even objects – they’re optical phenomena in the sky such as sundogs, ice crystal haloes, and the like. I’m a little annoyed at him, because his use of terminology was not correct – he called a lot of things “sundogs” that would not be designated so today. But I think his ideas were sound.
There are several cases that have been explained to my satisfaction by Menzel, Phillip Klass, Robert Schaeffer, and others. A lot of these investigations have been in great detail, not mere dismissals without thought. Klass devotes three chapters in his UFOs Explained to one case. Have a look at them.
Even most pro-UFO investigators admit that a great many supposed UFO sightings are the result of mistaken identity. You would certainly expect that, even if UFOs really were mostly alien spacecraft. The issue is how you deal with the cases where an explanation is not easily made, or there is insufficient information. The skeptics, of course, lean on the side of these cases, too, being mistaken identification, while the true believers lean the other way.