There is one, actually. In true open-source community fashion it has the delightfully easy-to-remember-and-say name of “XMPP”, because apparently “Jabber” (its old name) wasn’t technical enough.
The problem is, for reasons I outlined elsewhere, no one wants to use it precisely because it would allow people to communicate without being part of their own platform, and venture capital investors are all about those Monthly Active Users. Google Talk (the original Google chat app, not whatever reincarnation they’re up to by now) was actually built on top of XMPP, but they abandoned that in favor of inventing roughly sixty-eight proprietary communication protocols of their own over the ensuing 20 years.
Now, to be fair, XMPP as a spec does have its own list of technical problems that might well be solved by building something completely new, but nobody who has a say in how chat apps are made is willing to put in that work for everyone to then be able to use.
