Notice how the expression doesn't exist in a vacuum, it's a reaction. They'll sometimes use their hands or whole body to sell an emotion, and that part of their body is ALSO reacting. They aren't just flailing their arms in random directions to be funny. All the faces are treated equal; the artists don't ONLY go crazy for screaming. They put just as much thought into a shy face as they will a yelling face. Even for simple designs, there is solid construction. It still feels like the character and not just a morphing blob.

Compare that to something like pic related. The expressions are an after thought. You can erase the whole bodies of these characters and just leave floating eyes+mouth, and you get the exact same scene. They do not play with poses or warp faces around skulls. There is no consideration for if this is the right face for the right moment-- only that it is a FUNNY face for what could probably be a boring scene. You'll notice lots of artists will resort to drawing meme faces when they feel something isn't funny enough as almost a laugh track to the audience so they know, "This is supposed to be funny, by the way!".