Because even though he's the man of steel, he still needs to rest too. Besides, who really wants to be superman 24/7? By george, the man works a full time job at the daily planet, and he does the whole savior bit in his free time. the man needs a day off every once in a while. besides, why do you think he has the justice league for? they can always fill in for him on the days, he's taking a vacation.