Wow, what show are you watching, lol. They almost NEVER take their shirts off, especially Dean. In fact, you can count on one hand the number of times Dean has been shirtless or even in one layer of shirt! Sam has been shirtless more often, but not more than 5 or 6 times in six seasons. It's actually a running gag how many layers of clothes they wear! Totally the opposite of what you said.