They're just so... repetitive.
I mean, have you ever met a robot who can tell a joke that's not a variation of "I am the 0.5 megawatt laser, I am the 0.5 megawatt laser."?
Or, you know, just ask a robot to "Tell me a joke, robot." And they'll just respond with "Why did the robot go to the store?" "Because it was a 2-for-1 sale!" Ugh.