Because there's no chance of a win on either side. What's at stake is momentum going into the next match. While England keep batting, even if it's pointless in the context of this match, each run that England score makes OZ look more and more powerless. If England were to put OZ in now and try to get the OZ to go for a win, if they in turn can't take wickets then the questions will start to be asked of them instead.
So they will put the OZ in when there is not enough time to bowl us out, but where there is time to take a few key wickets at least.