Those have always bugged me. I recently came to the conclusion that it's because it's like putting on blinders to watch a video. I think it's part of why they're addictive. We are peripheral vision creatures. It's unnatural, like putting yourself in a box. I have nothing to back up that theory. 😅
Discussion
the reason is primarily that holding a phone horizontally is awkward and liable to result in a broken screen and to "help" you the screen flips to portrait display and cuts off the edges of the CCD input to further "help" you.
the CCDs are usually built 3:4 ratio and oriented in the same aspect as the device, so vertical with 9:16 is not chopping out so much of the available pixels.
some cameras have features that let you get all the pixels and just set a flag on the orientation, and then you have the problem that video players don't respect the rotation setting, and/or it doesn't support changing mid stream so if you start with vertical then turn horizontal the video is then sideways.
mobile cameras are the spawn of satan.