Buy an iPhone to use iMessage ? Some people are crazy.
Funny little fact: most of these systems all use email or the phone number in some way. For example as identifiers or for password recovery, etc.
Why do you think that is ? Because it's universal: everyone has one. Why is that ? Because it's federated.
Obviously, none of these services wants to be federated, because in Silicon Valley it's all about how many users you have (locked into your ecosystem). Doesn't matter if you are making profit or ever will, as long as you have enough users investors will throw money at it.