Because even the bigger stars could find themselves frozen out of any major movie, TV or theatrical work if they said anything.
Hollywood is like one of those old company towns in the middle of nowhere - the company runs literally everything. If the company screws you and you publicly complain about it, you quickly find you can’t get work anywhere and you start missing meals. If you’re lucky, you’ll have enough money to get away from that town. If not... well...
Yeah, it’s like that.
It is worth noting that James Woods has been frozen out of Hollywood for being an outspoken conservative. He has commented on this explicitly.
Yep, to work in Hollywood, you have to check your soul in at the door.