Standardized testing is a natural monopoly -- if you have every college making their own, it's a crazy duplication of work, will produce a lot of substandard tests, and will put tons of strain on the students. Might as well ask everybody to lay their own fiber.

ETS isn't perfect (I think the amount they charge for GRE-related stuff is sickening) but it at least does a good job of making sure the best prep resources possible are cheap or free.