<p>Because the AP tests don’t exactly ask a lot of you.
You can get a good score just by knowing the right tricks and doing enough practice tests, they don’t test your true understanding of the math and theories, your intuitive feel about the calculus.
With a school like Caltech, even its intro classes will be much harder than your typical high school AP course, and you’d be screwed if you tried to skip to something even harder.</p>