People are willing to deal with a lot of thing before it gets to a breaking point - whether it's a social media platform or anything else in life. Change is scary and many users keep their accounts to follow specific artists/series/topics who may or may not have easily findable alternatives to switch to.
Add the fact that majority of casual users doesn't know about other similar platforms or (in case of fediverse) finds them confusing and suddenly they have more incentive to just put up with the problems as long as they can. More so since such change requires rebuilding of communities and relationships.
Finally, there's laziness - most people just can't be bothered to change things in their life unless they have no other choice/choice was made for them (ex. service they used died).
Why do you think so many refused to leave reddit after the whole boycott drama? It sucks but that's how humans are. There's only so much we can care about and social media aren't exactly high on the totem pole.