But youtube's main services are free, so that's harder to pull off compared to stuff like Patreon. Offering exclusive videos probably doesn't outpace the ad revenue from "free" videos either (and if we're being frank, you're still bound to YT's rules. So you can't offer truly "extra" content free from censorship or copyright or whatnot.)
They do. But as explained, the revenue gained by maybe 100 users paying $5/month won't necessarily exceed an average video release of 10,000 "free" views. It's a " free service", so most subscribers (let alone unsubscribed viewers) won't join the membership for a few extra videos. It's a similar issue Reddit is trying to do right now with paid subreddits.
The idea can work, Nebula as a "competitor" works off this model. But I don't think it can be tacked on 20 years later onto a "free" service.