He may have been afraid of the penalty, but you can tell he was expecting Muller to shoot and guessed that he'd shoot high -- you can see him plant and spread his hands to block -- and got caught flat-footed when Muller put on speed and went past him. He had no shot at a dive after he stopped like that.
It's difficult to blame the keeper in this situation. The striker can do 1 of three things, shoot low, shoot high, or attempt to dribble. The keeper has to choose one option and attempt to stop it. He picked the wrong one in this instance.
I'm trying to find a highlight of the game that shows the whole build up of the 3rd goal ... Every single one I've found don't show Ribéry's backheel pass ...
Really the most beautiful thing about the goals in this game was the buildup. Each goal had a fantastic team effort, from Kroos' sexy through ball, to Alaba's fantastic run with 2 give-and-gos, to Ribery's fantastic vision, Mueller's great run and no look pass to the perfectly timed run of Lahm. We are actually playing attractive football going forward. It's a pleasure to behold.