Because people insist on discussing copyright as if there is any part of it that makes sense, and as if it operates how they think it should.

They derive a history of it from all of these principles that they made up, then propose a future which is always a moderate compromise between the guiding principles that they made up and the history that they made up from the guiding principles that they made up.

Things are as they are because powerful people made them that way, and built on that. The length of copyright is justified by the fact that it got past Congress and judges. What you're allowed to do is vague know it when I see it stuff, and has always been a patch on top of what you're not allowed to do which is always very clear: anything you don't have a written grant of permission to do.

People talk about "fair use" like it is a real abstract principle, rather than being some weird legal wording by a judge from a few court cases where something felt just too minor and silly to be a violation but was obviously, by the letter of the law, a violation.

I'm fairly sure that under the letter of the law you're allowed to read a book you own or listen to a record you own more than once, but I wouldn't bet on it. For all I know it could be an exception called "private repeat performance of licensed material" which is not a law but actually guidance written by the counsel for the Librarian of Congress based on two court cases from the 1930s.

edit: when I was a kid, you wouldn't put the song "Happy Birthday To You" in a movie, and you would edit it out of a documentary. This was never determined not to be a violation, it just got so embarrassing that it was somehow determined that the copyright had lapsed. Archive.org was in a years-long kerfuffle about 78s. It's not about sense, it's about power.