You're ignoring differences in the size of the space and the ability of most drivers to back-up vs going-forward into a small space.

>90% of the people who back in take much longer to back-in than they'd take to back-out because it takes them more time to back up with any accuracy.

Head-in to the spot is just as fast as head-out into the lane because most people don't have any trouble going forward.

The result is that back-in is a net time loss.