and the point of admissions is to weed out people who'll succeed in school rather than wasting everyone's time

I'd say that the point of admissions is less to weed out people who'll succeed from people who'll waste the school's time than to weed out people who'll reflect poorly on the status of the school. Colleges raise their status by taking better students, so their interests are served not by taking students down to the lower limit of those who can meet academic requirements, but by being as selective as they can afford to be. Schools will even lie about the test scores of students they actually accept, among other things, to be seen as more selective.