There's literally zero benefit that Apple, Google, FB or whoever is able to offer with their in-house implementation.
Some optimists here telling that all messenger might follow a certain "standard" for transmitting encrypted messages. And in fact some messengers might, but this standard will be the lowest common denominator, i.e. the weakest security protocol all know.
For p2p model generating secure private key is paramount. Can this be guaranteed by every peer operating in a chat? Not unless they all made by the same team with high security standard.
Or they simply could agree on solid common exchange protocol. There's no requirement to support crappy encryption. Just to open communication with your service.
We've dozens of functionality identical, incompatible proprietary protocols, because each corporate schmuck wants their own walled garden, to squeeze a few more cents.
Why should we agree to that?