Because if you assume that everyone is of roughly equal intelligence regardless of race, then each demographic will produce roughly the same amount of college bound kids, which will in turn fall roughly in with demographic lines.
Though with cultural pressures and social mores always present, you're not quite get that. And of course random happenstance will always keep things from aligning perfectly even without the aforementioned influences. What AA is supposed to do is acknowledge that sometimes society puts more undue pressure on certain groups, making it easier for some people to get ahead over others. At its best, AA evens out that cultural playing field, so everyone ends up succeeding by their own merits rather than pure chance, where and to whom they were born.
It doesn't always work out that way, but that's the point of it, and why I ultimately support it.
