1. Most programmers in the US go through 16 years of school. The final 8 have an effect on where you end up. Of the first four(high school) only about 1/3 of your time is spent on math/science* * . Of the final 4(at the two colleges I attended) only about 2.5 years are spent on math science. Of that 2.5 years only about 1.5 are computer-science specific.
This means you're determining somebody's programming competence based on information (What college and grades) that is less than half(48%) based on math/science proficiency and less than quarter(19%) based on computer science.
2. They're subjective. Especially liberal arts(i.e. MOST of school).* * *
3.It's incredibly expensive. Computer science and math cost next to nothing, but for some reason I'm supposed to give some school 60k over 4 years to spend about a quarter of my time studying what I'm interested in.
4.If you test out of a class it doesn't count for grades. If I work very hard and do 16 weeks of calculus in 3 and take a test, I get credit. If do nothing and just ace the class I get an A to pad my transcript. This creates a DIRECT incentive to do less.* * * *
* I am absolutely all for hearing about research/coding/contests I don't think schools opinions should matter at all.
* * I mention high school grades because high school grades determine which school you go to.
* * * Yes, math and science in high school/college can be and frequently are subjective as well. Teachers give credit for effort/showing work on problems/homework/etc. etc.. My high school algebra grade was a C despite getting 90+% right on exams because I didn't do the homework and the teacher wasn't satisfied with my explanations.