Most would agree that we are a very well coached team, so you'd almost have to believe that being so poor at throw ins is a deliberate tactic. Also, in the games I've seen, we take a very long time over throw-ins, which may also be deliberate.
Most teams that play a possession-based game seem to throw the ball backwards from throw ins and then start building anew. Not sure why it's so hard for us to do that, though you clearly need to do it quickly so your player isn't immediately marked. It might also help to have a left- or right-sided player who can throw the ball more than 5 yards in any direction.
Still, if throw ins are our biggest problem, we're clearly doing something right everywhere else!