The first world had a lot of computers, video cameras and horizontal screens in general before they had smartphones.

I think it plays a part.

The phone camera sensors often have a aspect ration of 4:3 but the sides are cropped in software. So the videos just get mutilated because convention.

Though at least 4:3 format is making a come-back because it is the prefect comprise format. Looks great on a tablet, is usable in both landscape and portrait mode. On Desktop it leave space to read comments. Perfect for youtube videos.

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 :)