Because it's TV, it's not real thats why indiana jones never takes a shit. Their oblivious to the "joke" in comedies because it's a script and their just following it. There probaley is a comedy where the actors laugh at each other. And why would actors see them selves on tv, usually their not even in the same country, perios of time or dimension, when they're ACTING. But to be honest I don't get how you don't get that TV isn't real simple as that.....

How about asking yourself if you've never seen in side your own body how to you know it's not all a sharade and that your chest contains 20 little smurfs controlling you?