It could be that most people's goal in life is not to watch the maximum amount of YouTube.
One thing I've wanted since I saw a prototype Joe Edelman made for Chrome, is after a video is played, prompt the user for a rating, and a reason for the rating. Joe had a taxonomy of human values for this, but it could also be a freeform tag based system.
Then when you go to YouTube next time, you say that the reason you're at YouTube is to "increase my knowledge of machine learning." Or maybe it's Friday evening and you just want to "make me laugh."
Most people probably won't pick "make me outraged or scared.", but that does give good engagement metrics...