demographics such as two-parent families, race, family income, gender, and introverts (I don't know that any/all of this is true, but just go with it).

If that's true, then basing the admissions decision solely on GPA/ACT score would eliminate diversity from the categories above. You could theoretically end up with the same cookie-cutter type of students. Adding the subjective element allows the admissions people to focus on other criteria that makes BYU a more diverse student body.

For example, having an African-American female with excellent leadership skills (but a 27 ACT) could be more valuable to the university than another socially-awkward white male with a 34 ACT score. We could debate that specific situation, but that is the idea behind it.