Not trying to be unkind here, but you keep bringing up examples from Oxford in a discussion about the American higher education system. There's a big difference in culture between the two, and so discussing Oxford as a counterpoint doesn't really make sense. It's not an American outlier, it's a totally different system with its own issues and culture that don't have much to do with this conversation.

And yes, schools in the US do care, deeply, about extracurriculars. Harvard could fill every seat, every year, with the valedictorian of a different high school class. Since nearly every data point is tightly grouped on the most immediately available metric, they turn to other data points to make their decisions. (And there's a long history of using that flexibility for both exclusionary and inclusionary purposes.)