well there are several reasons why in my opinion...
1) they are hot and sweaty, when I do sport I cant get my clothes off fast enough once I have to change, obviously not being a man I cant just take off my top outside!!
2) they work hard and have no personal body hang ups and people who are like that dont have any qualms about removing clothes, I dont see it as showing off but, that said if they work hard to get the body they have then they have every right to show it.
just as we all take pride in our own accomplishments, they take pride in the work thats given them a good body