If I was guessing, in general, I think people who advocate for a pure meritocracy in the USA feel the world should be evaluated in more black and white, objective terms. The financial impact and analytic nature of STEM and finance make it much easier to stratify practitioners "objectively" in comparison to finding, for instance, the "best" photographer. I think there is also a subset of US culture that thinks that STEM is the only "real" academic group of fields worth pursuing, and knowledge in liberal arts is pointless -> not contributing to society -> not a meaningful part of the meritocracy. But I'm no expert.