Because people haven't figured out, yet, that these platforms are nothing more than cliques, and instead of trying to impose law against privately owned companies, they refuse to just go make their own.
If everyone with any sense decided to just up and leave Twitter, it would be nothing but bots, celebrities, politicians, liberals, and idiots… But mostly bots and idiots.
See pics related. If you want to get uber famous and get tens of thousands of followers, just follow the algorithms.