Hmm, but how can they know? I mean, it's rare that your frequency of sweatiness coincides exactly with the daily cycle. Sometimes I can go three days without a shower, sometimes I have to shower twice in a 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.

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 also find a warm shower a pleasant way to wake up (and live in a locale where neither electricity nor water is scarce) - so it's a combination of sense, custom, and pleasure.