Pride

One explanation behind all these square wheels is the phase every programmer goes through during which he overestimates his abilities, has no sense of scale, and lacks sense for robustness. In short, he's proud, ignorant, and dangerous. He believes that libraries are bloated and slow, that he can out-perform standard implementations. He optimized prematurely, avoids function calls, abuses the ternary operator, and doesn't use a profiler.

Eventually, these programmers grow up, but in the meantime, they've written a significant amount of horrible code. I've seen this pattern again and again. As the parent mentioned, software developers have no "engineering culture." I imagine that in more established engineering disciplines, students have the above attitude beaten out of them before they graduate.