I think a big part of that is just straight up storage space, more specially a lack thereof. Google won’t release specifics but estimates put the total data stored by YouTube at somewhere near an exabyte (1 million terrabytes). Most of which is made up by video files.
Of course that’s just issue number 1 of many to figure out.
It’s a matter of finding where the line between cost and user satisfaction meet.
Like sure you could limit all videos to 60fps @ 720p or 30fps @ 1080p but most everyone now wants everything 120fps @ 2160p which takes up dozens of gigs per video and eats up bandwidth.