Recruiters and HR people are usually not subject matter experts, so they don't have the expertise to select great people from among outlier groups or from the larger population. They end up relying on indicators like top schools or top prior employers.

I've hired outstanding people for dev roles with all kinds of backgrounds including self-taught, low-mid tier state schools, unfinished college, etc., but that's because I am a developer and I can tell who knows things and who doesn't. I couldn't do the same for, say, a mechanical engineering position since I am not an expert in that field.