You are not training each muscle one a week.
When you squat and deadlift all of your lower body muscles are involved... YES the squat is more quads dominant and the deadlift is more posterior chain dominant, but in both cases all the lower body muscles still work and receive a training effect.
When you bench and overhead press all the pressing muscles also receive some stimulation. Granted the delts will work more in the overhead lift and the pec less... but in both there is still activity from the delts, pecs and triceps, enough to stimulate them.
The back is trained on the deadlift day but when you bench press properly it is involved in the bench also, in fact I often get a lats pump from the bench. And if you do loaded carries on both lower body days, the back receive some work there too.
Only the biceps receive some stimulation only once a week. But you can do a few sets on the upper body days if you want.