Marriage is a contract between the parties themselves. Why should they have to be married in the Catholic Church?

The Catholic Answers

Marriage is a public as well as a private contract, and society rightly insists upon public conditions for validity. And since marriage is also a Christian Sacrament committed to the care of the Church, the Church reasonably and properly lays down the conditions for the lawful and valid reception of that Sacrament.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty