Why do we need both phillip's head and straight screws?

I think this is stupid. Every time I can only find the Phillips screwdriver, I have a flathead screw, and vice versa. Once in a great while, I have a screw that some genius made fit either--it's a Phillips screw with one slot longer to accommodate a flathead screwdriver too. So then, why not make BOTH slots of a Phillips longer? Why do we need two types at all? Ahhhhhhh ! I guess "Why can't I keep track of my screwdrivers?" would be an entirely separate question :)