Already answered. Ghouls have no idea how their own physiology works.I too would like an answer to this.
Even in Fallout 2, 160 years after the war, ghouls were acting weirded out by the fact that the radiation from the GECKO nuclear power plant "made them feel warm inside". They STILL didn't know radiation healed them even after being ghouls for 160 years.
Dean Dominio in Dead Money also questions if he has to eat to stay alive. He literally doesn't know if he has to eat anymore, and simply does it out of the belief that he does. Despite the fact hes been a ghoul for two centuries, he has no clue if he needs to eat.
From the endings, not from a ghoul source:
The ghouls of Necropolis learn firsthand the final meaning of dehydration, as their city succumbs to the desert sands and the water runs out. Without their water purifying control chip, they do not survive