It just doesn't make sense that it can act on someone else's turn. Everyone is already acting on everyone else's turn. Everyone acts simultaneously, and it is only the turn structure which makes it appear otherwise.
Yes, but that structure is dependable. Folks make plans based on order of operations. Being able to act out of turn makes it *more* like everyone acting simultaneously, as it disturbs the player's ability to use that metagame fact to make plans.