Yeah, but they're carrying it in way that is (mostly) well-distributed relative to their muscular system. If I gain five pounds, I'm unlikely to notice it as I jog down the street. If I put on a five pound boot, I'm going to notice that.

I think something similar is at work with bike weight. I realize that a bike that's five pounds heavier won't actually slow me down all that much, but it will most certainly feel different as I ride it. Of course, if your bike is a tank to begin with it's still going to feel like a tank with a bit of a load, so beyond a certain point a little bit of weight probably doesn't make a difference.