Chat is still in its infancy as a technology. We are still trying to figure things out: what's the minimal necessary feature set? What should a server API provide to clients?
And a myriad of behavior/feature/privacy questions that different messaging systems handle differently, for example: what happens when I leave a group chat - should others see me leaving? Can I leave quietly? Should I have a choice?
Or: when I join a group, do I get access to the entire message history? Or: what happens when I block someone and then later message them - should the message history be restored? This and many other privacy related problems are handled in as many ways as there are messaging systems in wide use.
Let alone handling of audio and video calls.
Messaging and its feature set should stabilize as a standard way of communication before we can say to everyone: OK so now there's no reason you shouldn't use a standard protocol. If you don't, you are definitely a bad guy. Today though it's still not easy to make a case for standardization, in my opinion.