As others said they have both now. Main issue is the same with any other kind of charity: most people won't do it so it's a neglible factor without incentive (which makes it cease to be a donation in my eyes).
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.)
You're allowed to have content with unpublished links or not discoverable by search. I guess you could publish that content via email to sponsors that could obviously be forwarded, but that would be such a small number. I'm not familiar with peculiarities of subscription to channels as I never browse logged in, but do they not allow for videos to be visible only to subscribed users? Seems like that would be simple enough to do.
>but do they not allow for videos to be visible only to subscribed users?
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.
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.)