Matt Ward - engineer at Mux here. The simplest answer here is that most browsers and native platforms don't easily support RTP/RTSP these days. It isn't technically impossible to stream RTSP to say an iPhone, but it I'd say it is non-trivial compared to simply using AVPlayer with HLS. Our goal with this product is to make video easier for developers to understand and use so that they can focus on the other parts of their apps. We are running those rather complex streaming servers and we even provide some guides for how to get started streaming video to common platforms - https://docs.mux.com/docs/playback

If you have questions about video streaming on a specific platform feel free to reach out! (my full name at mux)