Because "chat" is pretty much meaningless as a term. The only common thread between chat apps is "bidirectional data transfer between at least two devices." SMS and Discord are both chat but have wildly different completely incompatible semantics, iMessage embeds full iOS apps and a payment network into the chat.
I can't see any world where chat gets standardized that doesn't involve throwing out everything except the most basic sms-style semantics which is basically what RCS is.
I can't see any world where chat gets standardized that doesn't involve throwing out everything except the most basic sms-style semantics which is basically what RCS is.