Why can men walk around with no shirt on?

Generally, they can't, but it depends on the setting. For instance on a beach this is fine, but to go to dinner with no shirt is not. In fact, a fine dining place wouldn't let a shirtless man in. Doing yard work without a shirt is fine, but to go to Home Depot for plants is not. It all depends on where and when a man doesn't have a shirt on.