Because there isn’t a good replacement. Mastodon is a mess with no reach and servers that will perma-ban you if you don’t post for a week, bsky is still kinda tiny, spoutible feels like “all politics all the time”, cohost is jank.
Nobody really left Twitter because Twitter is where everyone else is.
My anecdoral experience, although I was probably exagerrating a bit.
Still, if I take a break from twitter/bsky/cohost/spoutible/whatever for a month, I don’t come back to “we decided this account is banned now and you can’t get it back, have fun”. Lost three mastodon accounts like that before I just gave up.