Yes, it makes sense. I'm going to be contrarian here and say that apps should optimize for the overwhelmingly primary task that users want to perform. If most YouTube users are playing the video and then tapping the screen to expand the video to fill the horizontal viewport, that in theory should become the default. Playing a video back in a tiny section of a vertical screen is usually suboptimal UX.
But there are a number of valid reasons not to do this:
- They might have data that most users are stopping the video after a few seconds and going somewhere else; i.e., filling the video to full size is not the usual action taken after starting to play
- It breaks the heuristic of giving users freedom and control by forcing them to hold their device a certain way when they might not want to
- Making users hold their device a certain way is an accessibility problem, because not all users can turn their devices. Sometimes the device is mounted to something that doesn't turn. Sometimes the user can't turn their wrist.
It is good to question why things are the way they are, though. There was a time when http was the default protocol and suggesting that all websites should use https was a radical idea. Your favorite sites and apps looked a lot different before their owners started testing assumptions. Be the kind of UX person that challenges why things are the way they are.