AFAIK, schools like MIT (or my alma mater Caltech), are the exceptions that proves the rule. Single dimensional focus on academics (e.g., STEM) might be *one* way to get into an "elite" school that has a narrow focus, but isn't really going to get you very far in an admissions pool at Harvard, or Stanford.
Except that MIT sees thousands more applicants than there are spaces - and all those applicants have 4.0s and all those applicants have won science fairs. The applicant that has that and has also written/acted/played an instrument/etc. is going to stand out.