Med school in those countries tends to last a year or 2 longer.

Also most other countries have national education programs for secondary school. High school in the US is regulated at the state and local level, so it’s very variable. As a consequence, the first 2 years of college tends towards general and education.

Sure but in the US it is 4 (undergrad) + 4 (med school) so in total it is 8 years rather than 5 or 6.

Agreed regarding the US high school standards but students that take AP courses in high school can take the MCAT. They may not do the “best” but they will know some of the material. Schools in the US like UMKC have 6 year programs were high school students take the MCAT.

In a lot of ways, the first two years of US undergrad are "remedial high school". This also explains how, e.g. American undergrads typically declare their majors until mid to late into their second years.

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.