Because they're Genin... kids... barely-trained chumps that, in war, would equate to those nameless twats players cut down by the hundreds in a Dynasty Warrior game. Sure, ONE of them might have killed someone important or helped capture someone/something. But only with the help of a hundred other cannon fodder. I just find a Genin with titles to be screaming: "I'M IMPORTANT AND HAVE TITLES EVEN THOUGH I'M BARELY A NINJA RIGHT NOW!"

Think about it. Did ANY Genin from the show... have a title? Hell, did most of the Chunin have titles? How many JOUNIN had them, even?

I can't help but think that any Genin who openly claimed to have a title would be viewed as a self-righteous and arrogant bastard by other, higher-ranked ninja.