Because hiring is hard (and a time suck) and a degree from a good college happens to be a pretty good filter for whatever attribute makes a decent employee?
It's certainly a filter, but is there any real evidence it's a good filter? I would guess that is, but I really don't have much to back that guess up with.
Its cheap/cost effective is I guess why its used. Its also a demonstrable CYA. People are incredulous that a person can FUBAR shit with an degree from XYZ.
