Letterman is like the comic at the cheap bar. You have to be pretty drunk to consider his jokes mildly amusing, because if you were sober, you’d walk out of the bar and never return.
Letterman is not ‘funny’ but he thinks he is and his inebriated audience thinks he is. Other than being on the wall TV in all-night bars, I don’t know anyone who actually watches him.
Since I’ve stopped watching TV I’ve noticed that the masses are trained to think stuff is funny. They hear some trash and start comparing it to all the other trash they’ve heard over the years, the little “laugh” light goes off in their brain, and they laugh.