There's just not a lot to go on. The SAT score is one of the few nationally comparable objective measures, and obviously measures only a specific kind of aptitude. High-school GPA is objective but hard to compare across schools, and measures only specific things as well (ability to do well in high-school classes). Essays might give you more information, but might be written by someone other than the student. You end up falling back on some sort of evidence that the student has independent interests/motivations, whether president of some club, or webmaster of some site.
Also, in the case where you have a 2 students with perfect 4.0 (or better) GPA's, both got perfect SAT scores, etc- i.e., match eachother blow for blow, the only way groupthink people have thought of to get ahead is to have extracurriculars.
Ideally GPA and SAT scores would make the distinction for us. The top X% of students would range from 3.5-4 in their GPA, or something like that. Unfortunately, grade inflation and test coaching is compacting an increasing percentage of the 'top' students into the tiny tiny bracket around 4.0 and 2400 (SAT). Greater-than-4.0 GPA's attempted to extend the headroom, but much like raising the level cap in WoW this didn't help much.
> Ideally GPA and SAT scores would make the distinction for us.
You think these are good measurements of student ability? For myself, I couldn’t complain about such a system, because (probably like many/most here) I did exceedingly well by such metrics. But to be honest, I think they are absolutely terrible.
Problem is that every other metric is absolutely worse.
When you think about it, the whole idea of rating one human being as "better" or "more deserving" than another is pretty much absurd. But universities have a limited number of spots that they can devote time and attention to, and they have to prune the applicant pool down to something manageable. People are very uncomfortable with randomness, so if a university's admission policy was "We'll pick 1 out of 10 at random", the applicants would go "WTF?" So they make up some arbitrary metrics, add in some guesswork, and try to assemble an incoming class that they feel will be as interesting as possible.
It's sort of unfortunate that their rather arbitrary decision acts as the gatekeeper to something that lots of people want - a good, high-paying, intellectually challenging job. Chalk up another one to "life's not fair".