The buttons are (typically) on the front of the elevator. If they were on the back, you would have to move further to press them, and it would be harder to press them in an elevator that already had some people in it. Given buttons-at-the-front design, in order for everyone to face the back of the elevator, they would have to press the button for their floor, and then turn around, and then, when their floor is reached, turn around again.