Maybe they think the company can be a force for good or at least not evil, but it's not living up to that or taking advantage of its opportunity?
If everyone just quits companies instead of pushing them to do better, the only people left at those companies will be at best morally apathetic and at worst actively evil (in the eyes of whoever is trying to reform them).
By operating speech platforms, these companies are inherently political. It's not like they're making nuts and bolts. Inaction is still action. Letting people run ads with hate speech or building recommendation algorithms that start with mild conservative views and then send you down a rabbit hole that ends at full Nazi-ism, is still a political action even if you intended the platform to be neutral or unbiased.
And BTW - free speech does not mean Nazi-ism - in fact, the Nazi's had perhaps one of the most famous censorship ops going with Goebbels. It's actually a pretty good tell for fascism, dictators etc - make it hard for other voices to be heard.
When I read Trumps stuff, I just want to vote him out of office.