The same reason why people posting stories instead of actual posts. Or you really don’t want to write masterpiece everyday.
I think this is what was asked by a parent commenter: why enforce any limit (except for a sane ones) at all?
By enforcing a character limit you only allow a certain type of post to be made
Sorry but this even sounds wrong. You can write an eternal masterpiece in any form. Short story, a poem, a novel, an anecdote even.
In fact shorter form is more challenging. You have less room for a mistake. And lets be honest: most people are terrible writers|composers|painters etc.
This is one of the reasons you see threads and services that can present you threads in a more convenient form.
Yes, the one where all nuance and detail is lost after being trimmed to death so it can exist under the arbitrary limit and is much easier to misunderstand because the author couldn't put all of their thoughts in writing.
It does help with engagement though.
Some say Shakespeare was his (their?) best when he was limited to the fixed form of the sonnet.
... is this^^ the type of content you want on Itter? Because that's what you get from this crowd.