Yeah, I mean nobody uses SIP as an open protocol with email-like addresses. You could call me with an unregistered softphone. It would have been way cooler if I had any use for it outside of like two other nerd friends of mine who run personal Asterisk servers.
What do you mean nobody uses SIP? Most mobile calls these days are SIP based as SIP is part of VoLTE. If you have 4G/5G signal and make a call you can be sure it's a SIP call.