I know for me, the voice of Howard’s mom on Big Bang, made me laugh, because she sounded just like my friend’s mom, and said the same kinds of things. So you don’t have to see her, because so many people have someone like this person in their lives, and they are imagining the exact person they know. If they showed the actress, and she didn’t look like the person you know, it wouldn’t be as funny, the camparison would be wrong, and some people wouldn’t even make the comparison.
With the case of Seinfeld, it is actually funnier not to see the real celebrity, because the “mocking” traits of the character are not something the real celebrity would actually say, but are rather a caricature, and that’s what makes it funny. The real celebrity is similar, but the caricature is much funnier. Instead of having a lookalike actor, it’s easier and funnier to conjure up the real celebrity in your mind, and be hilariously shocked by the things he says, which are still similar to the types of things the real person says, only bigger and funnier.