I think it's largely just human nature. People don't know what they don't know, and those who "make it" without college are often particularly disdainful of academics. They may also be hesitant to hire those with a deeper background (which is a general phenomenon in many areas). In many fields, the percentage who can "make it" without higher education may be higher, but it's certainly do-able in our industry -- especially if they stick to a certain kind of software. Someone in this position may develop a real knack for avoiding hard problems (in the computational complexity sense) without ever quite realizing that is what they are doing. I almost didn't go to college, since I thought I knew it all, and had plenty of offers when I graduated high school. One of the most general things I learned was how much useful stuff there was to know, that I would have never fathomed.

In my experience, people are often hesitant to hire people who are higher on some (perhaps tacit) measure of nerdiness. Only when people want to win badly enough and it's clear that others are doing better, will many people start questioning such biases.