Note that this behaviour is configurable. It's default is a 500 char limit because that is the Mastodon default.
That's a shit default, and a shitty way to treat a standard that is meant for all sorts of communication. ActivityPub is not Twitter, and it is not just for short-form communication. In fact, a majority of the web is longer-form communication that isn't a mere 500 characters long.
Retorting with "this behaviour is configurable" is dancing around the issue. Good, sane defaults mean everything in programming.