Because of the type of game mode it is in a game about loot. Everybody has near equal damage potential to somebody running the reaper class, and sometimes can even outdamage a reaper based on weapon choice. Everybody can collect motes as effectively as a collector. Sentry isn’t really that much more powerful against blockers and invaders than everybody else. Anybody can invade and do just as good if not better as someone with an invader role.