It's because all of the classes are expected to be in the same party and all be played by equal real-life players. That's the basic premise of the game. It follows from that that each class should be able to contribute roughly equally, or you risk excluding a player from a large portion of the game. Not the character, the player- you know, those people who are sitting around the table trying to have fun with this thing. If spellcasters are awesome and everybody else is teh suck, then only spellcasters should be player characters. There are other good game systems based on this premise. D&D is not one of them.