I think because the most common and popular aspect ratio is 16: 9 they aim at that market and the 4: 3 displays get left out... but saying that , I have seen some strange resolution sizes in games lately...
I have a 16:10 monitor and don't really mind the letterbox. I really don't even think about it. Back in the Brotherhood days I was able to use the NVIDIA control panel to "zoom" in on the game, but it clipped both right and left sides off... I haven't tried since though.