- Removing the character limit would just make the feed unwieldy and unpleasant, especially when reading it in chronological order (bottom-up) like it was meant to be read. Typically a follower's feed will contain a collapsed version of the thread showing only the first one or two tweets and a "Show whole thread" button, so they can read a short summary in order to decide whether or not to go into the entire thing.
- Disallowing long text in threads would just push people into writing the first half of their post on Twitter and the second half on $THING (there were once specialized websites for this format, like Twitlonger), which makes the reading and commenting experience unnecessarily disjointed.
Another advantage of tweet threads is that people can share/like/comment individual tweets, which serves to surface the one or two interesting highlights out of a 30-tweet-long story. It's sort of like line highlights or line comments in some blog platforms.
(And if you disagree with my viewpoint, that's fine. I'm not intending to convince you to use Twitter, just explaining why things are the way they are to the best of my knowledge.)