EC's are a convenient means of filtering those students who likely possess some measure of desirability on that front-- who are well-rounded and socially mature enough to be good members of a learning community (inherently a social activity), and also to withstand the pressures inherent in an elite learning environment with positive coping methods.

Jeez, I don't know. I'm going to speak up for the loners myself. (I was not actually a loner in HS and managed to check some high-value EC boxes, though not nearly as many as some of my cohorts.) A lot of high school ECs are kind of...not so appealing to a very smart kid with a low level of tolerance for BS. Unless you are at a school which has the resources to have excellent music or arts (and that's your interest), or competitive this and that of very high quality (and you're competitive--I was and am not) or are in an areas with other great opportunities, perhaps you are spending your time drawing or coding or reading or whatever. Those can be positive coping methods, too. I don't think any of this means you are going to be a poorer student than Polly the Yearbook Editor.

This mode of thinking annoys my inner introvert. We can't all be president of Glee Club.