Why did the popes fight secular monarchs?

The struggle between popes and secular monarchs was fought for power. Many secular monarchs arrogated to themselves the right to appoint clergy in their countries, bypassing Rome. This did not suit the head of the Catholic Church, since all the clergy were subordinate to him. The independence of the monarchs in this matter could lead to the appointment to an important post of a person who did not agree with the position of the Pope. And the very powers of secular monarchs seemed excessive to the Roman See.