10 years ago, a person would be insulted for filming a video in portrait instead of landscape. Now, old landscape videos are being cropped and resized to fit in a portrait player.
I wouldn't mind quite so much if it was because the people filming that way believed it looked better. But the real reason is usually because they are too dumb to think of turning their phone on its side.
The horizontal display is meant to reflect our natural vision which is also a horizontal aspect ratio. There are niche scenarios like skyscrapers but the vast majority of the time it's correct to stay horizontal and frame the shot properly.
I've been videoing my daughter learning to swim. It's actually easier to capture it in portrait. Landscape would just have a lot more empty pool, for no gain.
Both have their place, though a lot of people are also idiots when filming.