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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.
As Kitsune Cavalry noted, organisations like Khan Academy are non-profits that are supported by donations and run for the public good.
To address your second question (why would you start a project that requires so much time and not charge anything for it?): the fact that the organisation is a non-profit doesn't mean that the people working there don't make any money. Part of Khan Academy's costs are paying \$12,000,000 in salaries to its employees. This includes \$548,116 per year (2014 figure) to founder Salman Khan. You can read more about Khan Academy's finances on their 2014 IRS 990 filing here.
