Because making a character suddenly look stupid for the sake of a once in a while gag is pretty bad. I've seen anime that did crap like that; main character was relatively intelligent, had a good head on his shoulders but would somehow manage to fall for obvious traps or suddenly have zero common sense for the sake of a bad joke. It certainly diminishes a character's integrity. You CAN make comedy work and not have our characters be complete bricks.

That said, you could play it out like they're just going with the flow and acting the part. Obviously not having a character that speaks leaves a lot open for interpretation.