The internal state of your body has a lot to do with how you smell, plus other factors like hair and genetics. The bacteria in your armpits consume your sweat and their excretions are what smell. The more contaminants in your body, the faster you will smell. It's kind of like if we eat garbage, our shit smells bad; well, when bacteria eat your sweat, if it's garbage then their shit will stink worse. My BO builds up a lot faster now that I eat meat regularly, compared to when I was a vegetarian. Maybe it's because meat creates more uric acid in the blood.
The other thing is that the more often you wash using soap, the more your sebum glands secrete oil to re-condition your skin and hair. A lot of people complain that their hair is way too oily so they wash daily, but really if they could suffer through an awkward break of about 1 week using only water (and maybe some vinegar to keep the scalp toned), their hair would require less washing.
Hair in general traps odor, which is why some people choose to trim / shave the problem areas. I think showering once a day is normal. In the winter time I don't really stink that much, but I just don't feel awake unless I shower when starting my day. It gets the blood flowing.
As for people who shower less regularly... most of the world does not shower daily. It's partly culture and partly economic. There are parts of the world where water is so precious that the idea of filling up a tub to bathe, or sit under water pouring over you would be absurd. Oil scraping and sand bathing are still common practices in many arid regions.
In the west, strict daily bathing came from the Romans, and then European colonialism spread the practice to their conquered lands.
