Egypt and Equatorial Guinea are clearly on latitude/longitude lines, but the rest look pretty random. I am actually surprised that it's as nonlinear as it is - go take a look at a map of Africa, not much is actually rectangular! There are a lot of straight lines (which would be another interesting metric to study-the countries most closely bounded by an irregular polygon) in several interior borders on the Sahara, as well as Namibia and its neighbors. But none except Equatorial Guinea are really rectangular.

The state borders, on the other hand, are often quadrilaterals. Perfect scores to Wyoming and Colorado, and near-perfect for many others.