Bacteria breeds in moist, warm locations. Providing more surface area only increases the viability of your pit as a bacterial breeding ground. It happens very quickly, and showering enough to prevent it strips your skin of natural oils.
The hair on my head is exposed to free air flow and is generally a low humidity environment.
Also, I've always been a swimmer and involved in swimming competitively. I have a bit of bias because of the culture of that.
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Pretty common in bodybuilding also, though not universal.