Socks that haven't been worn between the last time you've done laundry and the most recent time end up at the bottom of the drawer. So the worn socks get picked again and reworn before the ones at the bottom of the drawer. Then one week you skip laundry, everything ends up in the wash the next week and you realize that you have very different wear levels on your socks.
Also, wouldn't the wear work out to be roughly equal if you alternate socks roughly equally?