alot of muscles get trained indirectly so they get some work more than once a week.... triceps when you work chest, biceps when you hit back...shoulders get plenty of work with chest and back...
In reality, although you're targeting muscles directly once every 6 days, the shoulder girdle and shoulder joint as well as the elbows and wrists are being targeted 3 times every 6 days plus during squats and deadlifts on other days. These skeletal structures take a pounding everytime you pick up a weight in your hands.
Most debilitating injuries are joint related as these type of injuries take a long time to heal. I find it incredible that in almost every thread about training frequency there's hardly a mention about joint recovery.
Perhaps it's my age, but I structure my weekly training around joint recovery, not muscle recovery. I never do upper body workouts on consecutive days.
I use to train a muscle group 2+ times a week and one day I decided to try once a week. And my gains were larger because I was going so much harder on the weights for that muscle group one day a week