It does have them actually, but (somewhat understandably) nobody uses them.

Off-site schemes like Patreon are far more popular. That allows people to donate a small amount monthly, or a small amount per piece of new content. They get incentives (à la Kickstarter) to donate extra.

Patreon has addition appeal possibly because it doesn't make you entirely reliant on Google for your income. Even those making millions of YouTube get shitty support and could have their channel kicked off YouTube (or pulled out of the Partner Program) at a moment's notice for no good reason, with nobody to complain to if that happens.