It's because most people don't actually engage with the videos besides watching it, unfortunately for content creators on something like Youtube in order to be picked up by the algorithm and to actually maximize profits they need people to do the whole "like, comment, subscribe" thing or the whole system is simply stacked against them. So yeah, even on content creators that typically don't do that kind of thing eventually they might have too, hell I've even seen some breaking out the metrics like "Guys 60% of my viewers are not subscribed, so if you could it would massively help the channel."