Notice that even Eorzea days (or Vana'diel) don't perfectly align to real world hours.

If 1 game day = 1 RL hour, certain players who log in at regular hours would be forever limited to whatever time-locked content was available during that time.

By having game days slightly shorter or longer than RL hours, a player who only plays from 5-9pm every Monday would still have access to all cycles of the moon and all hours of the day over time. (this is obviously very important for FFXI where there was a large amount of time-locked content)