Cynically, it's also a way for the US system to gatekeep "poor" people from entering professions like medicine and law because of the extra tuition fees (and opportunity time-cost) needed to complete their studies.

I am a natural skeptic, but in this case I think it is just an accident of history how different systems developed.

FWIW, although this is not well known, many medical schools offer combined BA/MD degrees, ranging from 4-8 years:

https://students-residents.aamc.org/medical-school-admission...

When I went 20 years ago, my school did not require a bachelor's degree and would admit exceptional students after 2 years of undergraduate coursework. However I think this has now gone away everywhere due to AAMC criteria

In Australia, Medicine was/is typically an undergrad degree.

In the mid-90s my school started offering a Bachelor of Biomedical Science which was targeted at two audiences - people who wanted to go into Medicine from a research, not clinical perspective, and people who wanted to practice medicine in the US (specifically because it was so laborious for people to get credentialed in the US with a foreign medical degree, that people were starting to say "I will do my pre-med in Australia, and then just go to a US medical school").