I would hazard a guess that it's a result of the brain interpreting the electrical impulses before providing the "image" that we "see."
It does a lot of pre-processing, so to speak -- which is why we get such awesome illusions as the Poggendorff illusion [0], tricks like the Pulfrich effect [1], etc.
I don't know enough about the mechanics of how light enters and is interpreted by the eye, but I wouldn't be surprised at ALL if we "should" be seeing more like a fisheye lens but our brain is saying no, no, those lines are straight, based on the countless other stimuli it's receiving.