STEM grads do better because other majors no longer guarantee minimal quality control. No F's = No signalAn interesting insight. In this respect, part of the STEM grads' success is that, within majors, they are the ones most likely to perform as a crude IQ test.
In Theater, as an example, there is likely some normal distribution curve of IQs. Some of those majoring in Theater are deeply intelligent and serious about the subject, while others are skating.
In STEM degrees such as Physics and Chemistry, there are few skaters. The distribution curve is likely left skewed and with a higher mean. Nobody below a certain degree of IQ is likely to be able to major in Chemistry or Physics and therefore those majors serve as a closer proxy to IQ than would a normally distributed degree such as Theater or Communications or Sociology.