Their videos pop up in the feed by simply watching. That’s how 90% stays retained without an explicit connection. What happens after subscribing is two-fold: a subscribed viewer gets more “related” crap from other channels due to shown “interest”, and a channel gets more chances to get recommended to new people. So it’s a growth thing, but not directly linked.
But I agree in general that a problem may be sort of recursive or self-supporting and not always technical. I just see people often enumerating simplest solutions and defeating them in the same comment, so that nothing gets done or even tried, when real-world systems are always complex and complexity is hard but at the same time has many potential tipping points. People build empires on default settings alone and lose businesses on a tiny but stable leak in the funnel. Thinking that there’s no such important and easily overlooked points in any non-trivial system, and it’s just all about users, is a mistake.
As an aside though, only 10% participating isn't very unique to youtube. Its a pretty common rule of thumb that 89% only lurk, 10% participate in some limited fashion, and 1% become super active, in any online community https://en.wikipedia.org/wiki/1%25_rule