Because the top websites want user traffic too. If you come from a country that doesn't have IPv4 left the usual deployment scenario is a carrier grade DNS64 with NAT64. You're basically stuck behind a huge ISP NAT which makes quite a few things unpleasant and others downright break down.
By supplying v6 you ensure anyone in the world can access it regardless of which IP protocol is deployed by your carrier and how they are or aren't translating from one to the other.