Because it's not about being able to say what they want on a particular platform, it's about saying what they want where everyone else will hear it.

A lot of people look at this issue and see two groups fighting over it. There are (at least) three groups though: for, against, and undecided. This is really a fight about how to convince the undecided, so solutions like "go make your own platform" aren't desirable; undecideds aren't going to know about the new platform, and won't automatically appear en masse on it, so it actually makes reaching them more difficult.

If that's a genuine question .. .. .. you can research on the term network effects.