The crux of this problem in my opinion is that long term results aren't tracked.

It's hard enough internally to track someone's performance over the course of the year or two after they get hired, it would be even harder to do it if you are a recruiting company.

It's especially sensitive because employers are weary of sharing employee performance data to third parties because of the high risk of a lawsuit (there is clear precedent for these lawsuits.)

Once that data problem is bridged, it blows the problem right open for data to be explored and figure out what exactly predicts a top performer, in any field.