As I mentioned in another comment, the objective of elite schools is not to just admit 1600 SAT (or whatever the metric is these days). It's to admit "good" students and then to look at other factors. You have successful parents that went to the school isn't the only other factor but it's not a terrible one for both financial and other reasons. Neither is admitting students who didn't completely ace the SATs but also have other notable accomplishments.

SAT is a very very low bar to pass, especially Mathematics, it is ridiculously easy to get 800. Oxford and Cambridge basically have their own entrance exams that even the best students (eg IMO medalists) won't score full marks.