It also depends on your body's details. For example, my hair gets so oily if I go 2 days without a shower. I need to shower every day just to feel like my hair feels clean. If I'm going to do my hair, might as well do the whole body. If I'm in a rush though, I won't necessarily soap up the whole body every day.
I guess this is one of those cultural things, and people just do it half because it's good sense and half because it's the custom.