Pugsley: Are they dead?
Wednesday: Does it matter?
Gomez: Have no fear. The courts will decide. They say a man who represents himself has a fool for a client. Well, with God as my witness, I am that fool.
Gomez Addams: [admiring Morticia, who is sleeping.] Look at her. I would die for her. I would kill for her. Either way, what bliss!
Answer: Presumably when Fester was lost in the Bermuda Triangle, whatever supernatural forces caused him to lose his memory also made him grow hair. This gets reversed at the end of the movie when he's zapped by lightning, restoring his memories and destroying his hair follicles as well.