Schools like MIT do admit mainly based on academics. However, think about the kids that apply to these schools - many, many of them have top scores. Just having top scores is not enough. They have to look at other parts of the application. Here are some admissions stats from MIT:

They admitted 1,548 kids. Look at how many kids scored 750-800 in each section of the SAT, or 34-36 on the ACT. They could fill the class several times over with these perfect/near-perfect scoring kids. Yes, the academics are important - you need those - but you need something more to stand out.

These same kids also apply to Ivy League schools, Stanford, elite LACs, so at every top school you are competing against these same kids. Unless you are an athlete, URM or some other hook, you need to have something to make your application stand out. And even if you have a hook, there is still a minimum standard. A kid with very average SAT/ACT scores will not get in, no matter what the hook.

The standards have not dropped in the manner some have implied. I know kids with perfect scores who were rejected from some elite schools. The ones with perfect scores plus good ECs have better results.