Why don't colleges use weighted GPA's?

I think you are missing something. There is a "please note" information at the bottom of the Collegeboard profile and it says that its just preliminary info that it got off the responses from the universities. The colleges probably do consider AP classes and will look at them. Just because Collegeboard says to convert it to 4.0 doesn't mean the universities only look at regular classes.

Instead of whining about the GPA not being fair, move your focus to your SAT/ACT prep. Its another big part of the evaluation. Its true that grading scales can vary by high school, so universities don't really use the weighted GPA number. Pitt doesn't even publish an average unweighted GPA, but it says that many of the admitted students are in the top 10% of their high school class.

Do your best in your classes and apply to the colleges you are considering. College admission is based on a combination of academic and extracurricular factors, NOT just the GPA and SAT or ACT alone.

High schools all weight their classes differently, there's no standard. It's more of an internal ranking system, so that a school can tell where students are in relation to each other.

So, colleges look at the unweighted GPA, which is standardized. They will also take the rigor of your courses into consideration, and some colleges also look at the reputation of your school for academic rigor when selecting students for admission.

All of the answers so far are correct. I would add that weighted GPA's are popular since they give higher averages and that makes both students and parents happy. Most colleges also recalculate the averages so that they are assure uniformity. They also realize that say a 90 in one school or with a particular teacher may be different from another school or teacher..Bear in mind that colleges usually use about a dozen criteria for admission with a variation among colleges as to the criteria and the importance of each criteria.

They do not take your weighted GPA but they do consider the rigor of your course, so those who take all regulars but get A's but someone who challenges who gets A's and B's will look better. However, it's better to take a regular class and get an A than get a C in AP.