The SAT is not fair. By taking an expensive prep course, you can potentially increase your SAT score by 100 (although that's quite extreme, the average improvement attributed to SAT prep is closer to 20 points). That's a significant, but not earth-shattering improvement: it moves you from the 75th percentile to the 84th, or from the 97th percentile to the 99th.
Now let's compare that to other criteria used in the college admissions process.
It's way easier to have impressive and relevant extracurricular activities if you're rich and go to a good school. And unlike SAT prep, acquiring good extracurriculars will definitely take years of your time and money.
Well-written admissions essay? MIT requires one. But you do it at home, instead of a tightly controlled testing center. So if you're rich enough, you can have ghost-written admissions essays. Needless to say, this process can turn even a completely worthless essay into an impressive one.
Creative portfolio? Unless you're in the 99th percentile of musical talent, the difference between "I write songs that I play on an old guitar" and "an orchestra performed my composition" is money. The former is probably not even 75th percentile; the latter, probably 95+.
Alumni parents who would be likely to donate big bucks? You don't need an expensive prep course to get that, yet it can provide your application with a much bigger boost than 100 SAT points. And unlike the SAT, if you don't have alumni parents to begin with, then you'll never get this boost, no matter how much extra work you put in.
The SAT is not fair. But it's the fairest admissions criterion used by U.S. universities today.
(note: MIT still relies on the SAT; today's announcement concerns the SAT Subject Tests)