I left The Site Formerly Known as Twitter a while back, but you have to consider that not everyone on there is using the site as a source of world news. There might be individual people (e.g., artists) who you follow but only regularly post on The Site Formerly Known as Twitter. You might be one of those people, and The Site Formerly Known as Twitter might be the only way to grow and/or maintain your audience. Of course, I'd rather people leave for other platforms, but since many are on The Site Formerly Known as Twitter to follow individual people, as long as those people don't move, they themselves won't move.
It's the same thing as with the Reddit protests. Sure, people say they'd love to leave, but they're only going to actually do it if the communities they use also leave. A lot of people just aren't willing to suffer temporary inconvenience to eventually get something better. They'll talk about how they hate what X is doing, but at the end of the day, they don't care quite enough to use the less convenient Y.