In email you have sender impersonation. Once blocked, adding a new sender (cell number) is expensive. It is 12 orders of magnitude easier to spam with email than it is to spam with whatsapp.
But that's exactly Razengan's and my point. The email spam problem was solved even though email has very little identity verification. So spam can't obviously explain why instant messaging systems are so walled-garden incompatible, but email functions fine.
