Network effects. It's the same reason people are locked into twitter, even if they want to leave. They can't leave until the other people they depend on leave, and those people are in the same situation. A social network is a molecule, and your bonds will keep you from moving unless the entire molecule decides to move at the same time.

That's why Going Dark is an effective threat: it forces everyone to go somewhere else at the same time. Whether you want to exercise voice or exit, dealing with corporations requires organized, collective action.