Out of story, it's clear. Kings indulge in the final argument of kings so they have to leave their brides and any children alone, which allows the plot to continue by attack of the evil fairy, or the devil, or the wicked mother-in-law. You can't tell your son his wife gave birth to a monster if he can walk down the hall and see the baby. Or they go to war so that the hero can conjure up a suit of armor, some weapons, maybe a band of knights to lead, and save the kingdom, and win the princess's hand in marriage. Etc. Hunting also does the trick, often enough, but war has the advantage of taking long.
In the story? Well, they just go to war. Unless it's because the other king is attacking -- an excellent reason -- but that only pushes it back to that king.
Meanwhile, I've got a story where kings are attacking right, left, and center, and after some observation, I have to come to the conclusion that there needs to be two belligerent kings. However unreasonable his reasons for attacking, the hero's father is going to be a little miffed if his son leads the successful counter-attack. More than he was before. And relenting would look less plausible. sigh