Why have there been cases in history where a king divorced his queen because she would only produce daughters?

Usually it was an annulment.

They would not openly say it was the reason, but found some pretext (like too close family-relationships, or a former bethrothal that was binding).

Louis VII of France did this with Eleanor of Aquitaine, though they had other problems as well. Both would go on to have sons with a subsequent spouse.

Henry VIII did this with Catherine of Aragon and Anne Boleyn.