It's just a very loud part of the crowd. The other part, which includes myself, are control freaks. I personally pay for YouTube Premium, but for longer train trips I download everything using yt-dlp and view the files in VLC. I've been burnt a few times with the YouTube app deleting the downloaded videos after they've been taken down.

The other sideloading example is Telegram - some channels are simply banned by Apple/Google, but on Android it's possible to sideload the unfiltered app (even though RT is still banned there in EU, for example). Yes, 90% of those banned channels are irrelevant, but it infuriates me beyond anything that I don't have any control over my phone and can't decide myself what is available.