

Because cryptography is hard, especially when you’re trying to do it in a user-friendly manner, with syncing encrypted conversations between devices and whatnot. Like, it’s kinda the whole reason why the classic reply to “how do I make my own encryption algorithm” is “don’t”.
Also, with proprietary platforms you can’t make sure stuff’s encrypted the way they say it is
Except working without play services, that is, and some of us aren’t fans of having those around.