I highly recommend installing an extension that hides the home feed and sidebar recommendations, which at least makes YT non-distracting again.
Since a phone can show portrait or landscape videos in fullscreen (just hold the phone vertically or horizontally), it makes sense to shoot in whatever orientation fits the content or situation best.
The real problem is that computer monitors don’t easily offer orientation switching :)
I'm with you there. It's the same for shooting still photos.
...but that doesn't stop people from shooting portrait video and then constantly panning back and forth because the whole (crowd, landscape, giant sea monster, whatever) doesn't fit in the frame.