Forum:Why do we need to have attachments for things already on the gun?

For example, in Zombies when you buy a gun from the mystery box  or off the wall (provided it has a full auto/semi auto function in real life) you can clearly see the select fire or sometimes even a folded grip or adjustable stock. So why, in multiplayer, do we need to waste an attachment space to have something already fully equipped on the gun? Not really an attachment or a modification, but rather an adjustment.