Here's why:
* The Prophecy of the Three: The prophecy foretold that the children of the Big Three would be powerful enough to tip the balance of power and cause a devastating war. This prophecy was why the Big Three were so hesitant to have children in the first place.
* The Death of Luke Castellan: The death of Luke Castellan, the son of Hermes but raised by Zeus, was a stark reminder of the dangers of having children.
* The Titan War: The threat of the Titans returning and the possibility of another war was a major concern.
* The Need for Peace: To prevent another war and secure the safety of the world, the Big Three decided to abstain from having more children.
This pact was a difficult decision, but it was made to protect the world. It also had a lasting impact on the story, as it contributed to the development of the main characters like Percy Jackson, Annabeth Chase, and Thalia Grace.