There are two really simple reasons. One is that any such collusion would itself be a breach of anti-trust. The other is that they can't. Facebook has its own servers in its own data centers, its own fiber lines, etc. They could remove the apps from the iOS and Android app stores, but that wouldn't affect the billions who already have the app or get it preinstalled by carriers. And that's all beside the question of whether Facebook deserves such special treatment, either compared to the others or at all.