Any good university will have a limit to how many students it can absorb. A professor can't teach 1000 students in a class, not well. So you either have a university that can accept ~any number of students and function as mostly a diploma mill; or a university that actually cares about teaching students and thus must have a selection process.

Of course, there is some room between these extremes, especially for unpopular subjects where you hardly even get enough students to fill up a professor's time. And in those cases, you'll also see that EU systems will essentially accept anyone. Typically, for uncompetitive universities and subjects (majors), the only condition is to have passed (gotten at least 50%) for the local equivalent of the SATs - a very low bar.