...even if the government wanted to do it, because so many years have
passed.

Remember the final scene from "Indiana Jones: Raiders of the Lost
Ark", where they wheel the crate with the Ark on it (stamped "Top
Secret" of course) into the warehouse full of hundreds of similar
crates to hide it? So, guess what the Government does with UFO stuff.
It gets hidden away to help keep it secret, but then more items get
stashed away in the same building over time, things get rearranged,
moved, and forgotten. So after, say, 60 years for the Roswell incident
for example, it's been buried so well nobody would be *able* to dig it
up even if they wanted to! Heck, in that time, anyone who might have
had an idea as to where it was could be DEAD by now! Uh oh...