You seem to think the amount of prep you did was minimal. I'd wager it's more than most students do. It's certainly well in excess of anything I did to prepare and while my score was lower, it was more than enough (along with grades, activities, etc) to get me into my state's top public university.

The test prep centers do basically what you did - they just cost more and hold the student accountable (in as much as the student spends time at the center and presumably works through the test exercises).