I think it's just inherent in the personality types that are successful at software. We are good at understandinging complex things, and we like it; it makes us feel good to master them, and creating one is a special variety of mastery, so we incline towards that.

I think it also takes experience to see that simple solutions can be more robust; the more natural response to foreseeing future problems (another trait) is to have specific responses for all of them, which generates a complex result.