1) Because they're DBs... especially in this day and age with all the spread offenses, schools are keeping all the guys with hands on the offense side of the ball that they can.
2) Because they haven't been in a good position to catch it, even if the player is in a good position for the coverage, most of the balls have taken bounces outside the DBs framework, and while they still might get a hand on it... it'd be a great catch if they caught it (and they're not great catches because see #1)
3) Because they want to frustrate us now... and pick it up later in the season... notice how the groups are slowly improving? At some point they're gonna get hot and go on a good run... and the main concern will be that them getting hot will be too early and they'll cool down by the time of the playoffs. So slow steady burn, and get hot right around week 15 or 16, and maybe they can stay hot into the playoffs.