

Blame the Mastodon team, if you’re not running a fork, you have to go into the source and adjust the character limit manually.
Nobody has to do it like this, Mastodon supports longer posts since other servers and clients support more, it’s seemingly just a choice from upstream.
You need to wander through links a bit to find it but https://ofourdan.blogspot.com/2023/10/xwayland-rootful-part1.html talks about the rootful and the recent changes.