"Weed out" is a euphemism. The purpose isn't to diminish the number of people following a pursuit, it's to ensure they have the analytical aptitude to be successful at it.

Quite a few competitive major programs where I got my degree required students to complete 1st year calculus (differential, integral, multivariate) and some or all of 1st year physics (mechanics, electromagnetism, waves, optics) before applying.