Even disregarding the orientation, people hate auto-next on YouTube, but will tolerate/accept endless scroll for shorts, especially because they're short.
When smartphones were still a new thing, I kept preaching to take videos sideways because all useful playback devices were sideways. Man, how that has changed...
Most people I see are hugely addicted on doom scrolling content and shorts are part of it.
If you haven't hade use of selecting "not interested" on videos. You should now. I barely get brain rot recommended to me but sometimes the alg is very persistent.
pretty simple. [the video site] sees how popular tiktok is and wants to get a slice of that "short format" pie. It incentivizes content creators to make content that fits the format by promoting it on the algorithm. If there is a shortage of content in that format (and for that type of audience), anything semi-decent will do, eg. highlights from longer videos. As the algorithm needs to fill that quota of videos somehow