They did try it with Google Contributor, but only for a short amount of time to a limited set of users (US only). Now it changed to be for specific websites only which is pretty absurd.

I believe they haven't move toward ad-free plans simply because it would cannibalize their own product. They can sell ad space because they get ad view from all type of consumer. If they can't reach theses consumers, they have nothing to sell, thus will lose theses sales.

I think it's absurd though because it's the future and instead each website will implements it (and some will implements it together) which will cannibalize Google. Patreon is just the beginning.