Structure of the test goes back to the earlier point. Vocabulary and 10th grade math are things all students in the United States should be familiar with. Sure, they could test more complex math skills related to the type of methods work you will do in graduate school, but how many individuals actually take a stats, calculus, or even college algebra course? There is too much variance at the national level, and then when you add in international students, variances goes through the roof. The current test is far from perfect and needs a major overhaul, but at the same time, can you fault programs for looking at the GRE so closely? With that said, I do think a subject test in political science would help, but given that the discipline has become so quant focused over the past years, it probably would be more like the current math section in the GRE due to the aforementioned variance in undergraduate education.