Because dht doesn’t gather there. Dihydro testosterone.

Also, we do lose some hair density as we age. The beard usually doesn’t, but the pubes often do. But that’s more a matter of it growing back in slower as the body slows down. I’d say it’s an 8/10 thing where’s there’s visible decrease in pube density.

Used the be a nurse’s assistant, so I’ve seen plenty of naked people lol.