In theory, the point of having medical students have an undergrad degree is that they will be more "well rounded". Unlike in many countries where university education is very focused, in the US, an undergraduate even studying STEM is required to take a certain number of courses like literature or history, and likewise someone studying literature or history is required to take some science courses. Of course just requiring people to take courses doesn't mean they will actually retain the knowledge after the tests, but it's a nice idea anyway.