There's countless proxies in the wild that would not behave correctly with an RFC-defined GET-with-body, and there's no way for a client to know if that's the case.

QUERY has the advantage of getting default behaviour from most proxies (which at least is well behaved even if inefficient). If there are any proxies that just drop QUERY requests, at least they won't silently mangle the request.

This is the same way that instead of improving how HTTP 301 was specified, HTTP 308 was created. It's a pragmatic move.