Because it won't kill you to change up your wardrobe once in awhile. Don't get me wrong, your uniform has its...[ahem.]...charm. But what's wrong with dressing like a civilian once in awhile?

Besides...think of it as a souvenir. We came, we fought evil, we got shirts for proof. [And if he happened to buy Midnighter's a size too small well...he gets something out it too.]