Why dosn't the dog just walk?
Leashes imply that you're too poor to afford a dog that doesn't need exercise. That, and people like pointless gadgets that torture their animals by depriving them of the freedom to investigate their world. It's all about having a dog, but not looking like you care that you have a dog. Dogs are an accessory, like a purse or an expensive pair of sunglasses.
That's my guess, anyway. Now if you'll excuse me, I'm going to go attempt to re-balance the universe with my dog and her beloved tennis ball.