Because it's no big deal for most guys - most guys had to do it in gym class and sports in school, had to do it in dorm and military settings, etc. It's just not a big deal - I see guys naked all of the time in the locker room, steam room and sauna at my gym - although technically they are supposed to wear bathing trunks or a towel in the steam room and sauna, some guys do some don't. No one cares as long as they sit on a towel for sanitary reasons.