The rationale for meritocracy is that a limited resource should be allocated to the individual that will make most use of it. Demonstrating that you understand the prerequisites and are studious is a pretty good indicator that you’ll attend class and strive to learn more.

Randomly picking students with zero minimum qualification would be a massive waste of resources.

Randomly picking students above a given objective standard would be okay. Though arguably not as good as given the best of the best first dibs (depends on who you ask!).