Because it is contained in Scripture, "Behold, I lay in Zion a chief cornerstone, chosen, and precious: He who believes in him will not be disappointed."
A chief corner stone ...  The type of stone meant here is not the kind usually called by that name today.  "It was the stone at the extremity of the angle which controls the design of the edifice and is visible."[22] In the church, Christ is both the foundation stone  (1 Corinthians 3:11) and the cornerstone.