Tell that to Shilton and other keepers who tried such strategies....penalties are basically a game theory which thrive on unpredictability...if the keeper becomes predictable it is very easy to score once you figure out to just hit it at a fair angle ...Furthermore the reaction times between looking where the ball is going and acting are too slow to keep the ball from going in. Unless players decide to be too clever...the whole choosing a side and throwing yourself is a much safer bet.
1st rule of PKs, the keeper should never be able to block them. A perfect penalty strike is unstoppable. A pk is a situation in which the Keeper is at disadvantage at all times. Therefore he must do everything in his power to reduce the disadvantage.
There are three schools of penalty stopping. The first is the traditional school of penalty stopping, in which the keeper picks a corner to cover and a location (high or low) and goes for it. In this situation the keeper is almost always at a negative advantage so he uses certain tells to determine the direction of the shot. These tells range from eye contact to the corners, direction of the hips, angle of the ankle, angle of approach, and so on and so forth. The only problem with this approach is the evolution of the penalty taker, thanks to CR7 and other classless pussies keepers now have to contend with head fakes, fake run ups, fake shots and a whole long list of distractions before the shot.
The second school of penalty stopping is what I like to call Jelly-Legs University. Here keeps are taught to girate up and down the line in an attempt to throw off and distract to spot kicker. It works occasionally (Dudek 2005) but mostly ends up with the keeper attempting to reverse his position and force a reactionary save.
The third school is what I like to call Cheaters School. It's basically what Guzan and other keepers like him do. It requires to keeper to be quick and have above average athleticism. Basically the keeper steps off his line as quick as he can as soon as the ball is struck or in other cases just before the ball is struck (see Taffarel against France in WC 94). This forward momentum gets the keeper moving and makes it a much easier save because the body is already moving and not coming from a dead stop, and in addition the angle is cut and the shooter has less goal available to him. All this is very possible as keepers generally have better reflexes than us mortal souls. But keepers with little athletic ability are less likely to be successful with this method. And of course if the shooter hits the ball perfectly there is no chance of stopping the shot because after all, it is a penalty.