Cause we (well I and most that are recommending only 3 or less days a week) train at a higher intensity and different metabolic zone than those daily workers. For instance my family has a lot of iron workers in it.... lots of heavy lifting right? Well kind of. Its more like your working in the muscular endurance realm than strength or power. Your endurance fibers tend to recover more quickly than your fast twitch ones associated with higher 1RM and such.