Note #1: Although the Underworld is considered barren, it is not. There are plants that seem to grow there, and this tenuous basis would be a pretty good reason why Celeste was born. The God of the Underworld and the Daughter of Spring having a daughter would not be impossibility considering their essences and where they are, but the process would be extremely difficult, hence why only one Child.
It would be far more unlikely for Thanatos himself to have a child, hence Celeste as his replacement.
The evolution of belief systems and religious faiths would also play a part, but as the Divine Being’s area of effect and existence are regulated only by the faith of their followers, it makes more sense for them to be regulated back to being powerful mortals in various guises or taking just one long-term and divine appearance as an immortal.
Hollywood needs its muses to be less motivated by the shineys and more by the fundamentals.
– Note of Naoko’s Author
Keys to Twilight Reasoning: Examination of the Divine Beings and their status are regulated by faith