All I can do is link you to the help guides
https://help.twitch.tv/s/article/broadcast-requirements?language=en_US
https://help.twitch.tv/s/article/guide-to-broadcast-health-and-using-twitch-inspector?language=en_US
Twitch clearly has a reason for running CBR on computers, and adaptive bit rate just has to be done on mobile due to the spotty nature of cellnet data coverage.
But this isn’t really a question for this forum, as
- this sub forum is for discussion and support of analytics related to extensions and mods provided via curse forge/twitch app
- this forum is for support of the various TwitchAPI’s (non streaming) for third party developers.
I wouldn’t call OBS/XSplit “advanced” myself.
You are in the wrong place for such a question and are unlikely to get an answer from Twitch about why they do it a particular way since it pertains to how the transcoders are configured.