The queen and king of Ashland were loved by their people, but after years of being led by the couple and there were no signs of them producing an heir, slowly some noblemen in the capital were concerned about the future of the kingdom.
They started talking about the throne's successor. Heirs were very important for the sustainability of power.
If the monarchs didn't have their own children to be the rightful heirs, then it was likely for the powerful lords around them to fight for power after the king and queen died.
It's true, Raphael and Rowena didn't need to worry about their deaths. They were immortals who could lead Ashland for hundreds or even thousands of years if they wished to. However, they didn't want humans to know they were immortals.
It would create confusion. Humans might even worship them and Raphael knew it would greatly offend the gods in Cretea who were used to receiving human devotion and worship.