I'm guessing (say) Ivy dropouts have SAT scores in roughly the same range as Ivy graduates but are less employable because of the whole thing of a degree showing that you can stick with something and do what's asked of you for 3-4+ years. So it isn't solely a proxy for asking IQ/SAT scores.

(I'm a dropout, though not from an Ivy, and that's definitely something I've had to do a lot of explaining of in every interview I've had. Haven't been on the market for a few years; if there really is a magical age where people stop caring about degrees, I look forward to reaching it.)