Because judging the quality of "free thinking" outside of STEM is incredibly biased and subjective on the person doing the judging and could even get you in trouble for wrong think (try debating the Israel vs Palestine issue and see), which is why many school systems have converged on standardized boiler plate slop that's easy to judge by people with average intellect and training, and most importantly, easy to game by students so that it's less discriminatory on race, religion and socio economic backgrounds.