Because you don't want it to be too wide.
I carry about 16 cards of some type with me at any given time. Obviously, some of the cards are going to have to be covered by others, otherwise the wallet that I have would be ridiculously large. Of the 16 cards in my wallet the only one you can see entirely is my driver's license. My main credit cards end sticks out from where the money clip is, but other than that every other card is in slots with at least one other card.
Man, pickpockets will have some fun with SOMEBODY :p Jeff, may I ask what kind of wallet that is? I have one but it's so bulky and the leather is crap.