The same people were there from the beginning. They catch every act which is always the same and many of them know that act and can repeat it verbatim.
So why do they cheer as if they are hearing it for the very first time?
They love the tired old comedian and don't want him to feel rejected. So they yell and cheer and applaud and stomp and high-five just for him. In fact they are the entertainment and he is the audience. And so it has gone for a very long time and so it will until he stops which is when they will stop.
