RTSP doesn't go through firewalls easily. That's pretty much why Apple invented HLS. It's a nice, minimalist solution to adaptive bitrates that uses standard HTTP to get the job done. Big players such as the BBC have adopted the standard (ugh, I'm writing a client for it right at the moment!), so it's not a particularly fair categorisation to label this as a purely Apple thing.

If it's possible why hasn't anyone implemented such a thing? I'd like to implement such a thing and I wouldn't know where to start, but I would with HLS. This is the 1990's all over again. Different browsers supporting different standards, different video encodings, different JavaScript features, sometimes, when I'm having a spell, I'm not so sure competition is actually all that great anymore.

And no you cannot stream with webrtc without building a complicated back end webrtc MCU implementation which requires significant engineering and is not at all supported by the creators of webrtc who are focused on peer to peer inside browsers.