The otherwise sensible people I know who are still on Twitter all say it’s because of a specific interest or group, and the community of people around it who are all on there as well. They all hate what it’s become but put up with it because nobody is sure where else to go.
There’s also a sense of FOMO when it comes to realtime news updates. Until government, news media, and personalities go somewhere and take all their followers with them, it will be hard to break away.