The main reason is flexibility.
One day your boss stands next to you and asks: "can you move this border three pixels to the left", and the only correct answer is "but that requires a complete rewrite".
One day your boss stands next to you and asks: "can you move this border three pixels to the left", and the only correct answer is "but that requires a complete rewrite".
I don't think anyone was ever able to talk reason into them.