In spite of being one of the most hilarious voices in The Big Bang Theory, why was Howard Wolowitz' mother never shown?
As a writing device, it's far funnier to let the viewers imagination run away with the wild descriptions that Howard gives. Additionally, by not showing Howards mother, the writers are able to characterize and describe her any way they like, without having to find an actor who lives up to that description. Howard repeatedly describes his mother as having no neck, facial-hair that needs to be removed, and at one point, it was mentioned that she wears a wig. Howards mother is essentially a charicature of an overbearing, overprotective, troubled but caring, single-parent.
Now imagine if we were to actually see Mrs. Wolowitz's reactions when Howard yells at her. Perhaps we would start to feel uncomfortable witnessing this coddled, well-off guy in his 20's, yelling at this morbidly obese, lonely woman. By not showing his mother, the audience can separate the charicature of the woman from the far less funny "reality."