In a penalty situation, the ball takes less than 0.3 sec to travel from kick to goalkeeper.
There is no time for the goalkeeper to watch the ball and dive in that direction.
He has to make a decision before the ball is kicked.
We have seen many times in penalty shootout, the kicker kicks the ball right in the center of the goalpost, and yet we have seen the goalkeeper diving on the right side or left side.
Even the goalkeeper knows well that standing in the middle would have 33% of chances to save the goal, but still, they dive.
The answer is: 'Appearance'.
It looks more impressive and less embarrassing to dive to the wrong side although, than freezing in the center and still watch the ball sail past.
This is called 'Action instinct'- look active, even if it achieves nothing.
The 'action instinct' comes in to play when a situation is new or unclear.
In new or shaky circumstances, we feel compelled to do something, anything.
We as a society at large still prefer an action that doesn't yield anything rather than remain sensible and do not act.