Why pull out?

No, this post is not referring to the bedroom. It's about bowlers who decide, in their infinite wisdom, that they will decline to bowl the ball because the batsman has made a movement at the crease. Writing the previous post reminded me of the occassion when that happened in the World Cup when Fernando pulled out of his run up on the last ball with England needing 4 to win and Bopara shuffling around the crease. And it seems to have been happening occassionally in the current England v India series.

Surely, if the batsman is moving around, all the advantages are with the bowler. The batsman's head is moving around, making it harder for him to play the ball as he wants to, and if the bowler has time to pull out of their run up, then they clearly have time to readjust the line and length of their delivery. If the batsman has moved outside leg, just bowl a yorker relatively wide outside off-stump. So, why do bowlers pull out?