people hold the phone vertically when shooting video because that’s the way the video looks when they watch it on their phone. This is the most correct way to prepare a video for casual viewing.

People hold the phone vertically when they shoot videos of other people, and other people happen to be vertical. They can see each other (aspect ratio)^2 times larger: more pixel, and bigger on screen.

No they don’t. People hold the phone vertically when shooting video because it’s the easiest ergonomically to do one-handed.

My GF watches youtube vertically... only using 1/3rd of the screen.

So do I. It's a little annoying, but 99 pct of the time I'm holding the phone vertically, and rotating the screen is usually not worth it.