How come websites such as Khan Academy and Anatomy Zone do not charge for their services? I've noticed that most educational websites do not charge for their services, even if they are maintained by only one or very few people.

Some of these websites are so comprehensive that they require almost a full time job to maintain.

Why do these websites not charge for their services? (I doubt they make enough money from youtube to make it profitable).
Khan Academy is a 501c non-profit. They do some fundraising, they have some regular backers, and some big time contributors like these.

I imagine it's a similar case for Anatomy Zone. They are affiliated with a bunch of other non-profits that you can see on the bottom of their front page.