Not really. It depends on the state you live in; in my state, there was an exam you had to take in order to graduate, but it was given in 10th grade instead of at the end - didn't make much sense, except maybe to allow those who didn't pass the first time to re-take it without graduating late. So overall, I'm not aware of the US having anything similar to the GED that students take while actually in school. So there isn't anything to pull from, in that sense.
Of course, your original question was: why privatize it? They probably did it because the college/university entrance exams have been privatized for a long time, and those seem to be okay, so why not? Sounds kind of lazy, but I don't have any better ideas