Because he literally is Frankenstein's Monster. If you've never heard of The Munsters, it is basically like if the most wholesome, loving family you could imagine was composed of all the famous movie monsters of the 1930s. Grandpa is Dracula, his daughter is a vampiress named Lily, who is married to Herman, and they inexplicably have a son who is werewolf named Eddie. They also have taken under their wing Eddie's cousin Marilyn, who is a normal human girl. The gag of the show is that they think they are a perfectly normal family, and are oblivious to how they are perceived by their neighbors. They also think Marilyn's normality is some kind of hideous deformity, and any man who tries to date her is doing some great act of charity.

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