To answer two questions...
Extracurricular activities are important for getting into selective colleges (our informal word for unis) because US universities have a weird idea that they are looking for the "best people" and not simply the "best students". Extracurricular activities are one way to show your character and widespread interests and abilities. Colleges are also trying to create the "best class" (which is not just a collection of the best students, but ensures diversity goals, that the orquestra has a tuba player, a mix of artsy and STEMy students, etc.) which they believe will enhance everyone's college experience. Believe me, it's weird to me too after working in another country's university system which just uses a formula of an entrance exam and high school marks. Strangely, it seems to be related to anti-semitism back in the 1920s when Jewish students were set to overwhelm WASPs at elite universities, so they began to look for "character" as a subjective way to weed out Jews in the application process.
You can't go to law school, or med school, or dentistry school right out of high school because you need a undergraduate degree first. This change was made decades ago (a century+ in some cases). So technically you can apply for med school after majoring in art history in undergrad, but obviously it will be easier for you score high on the entrance exams and get in if you studied something relevant like biology. The why of this is a bit misty since it's so long ago, in some cases it was about saving a classic undergraduate liberal arts education (there's a captive audience if you force all future lawyers, dentists, social workers, etc., to get a different degree first), in some cases about artificially inflating the credentials of the professional (I've heard this about pharmacy school). The system is so old no one questions it, really.