Hair has 3 different phases in its life cycle. First the growing stage, followed by the resting stage, and lastly the final step in which it sheds and waits to regrow.

The hair on our head has a very long growing stage (several years) and then a shorter resting stage. So the body knows that hair will grow for x amount of time before tapering off.

Body hair, however, has a very short growing stage time (days) and a longer resting stage. That's why it grows in quickly but never gets long