From personal experience, my impression is that this kind of defensiveness comes from a lack of perspective. Especially when you realize you're hurting someone (and perhaps yourself). I don't know whether it's confined to nerds -- there are a lot of regressive, defensive people out there. :-)
For people who pride themselves on being analytical and meritocratic, it can be a difficult thing to realize that no amount of detailed analysis is going to help because you've been living in a bubble for most of your life -- you have only a small amount real data to work with. It can be even more startling when your meritocracy not ends up not only not really being one, but possibly impossible to achieve given the constraints of human nature.