I think this is largely a result of the average CS student never working on a non-academic project. The projects that CS students tend to work as coursework don't require too much investment in learning to become a good programmer--the code they write is written, turned in, and never looked at again, and working in teams typically isn't common. They never have to learn to be good programmers. The exceptional one's seek out extracurricular work and their programming improves with practice.
