Because the Catholic Church is One, Holy, Catholic, and Apostolic.
Ten years ago I belonged to an Episcopal congregation that called itself “Biblically Orthodox”, in contradistinction to more theologically liberal groups within that denomination, and made the mistake of asking what “Biblically Orthodox” really meant. To find out I went to the Fathers of the Church, Clement of Rome, Ignatius of Smyrna, Irenaeus of Lyons, and Justin Martyr, men with a link to the apostles. And what I found in their writings was the Catholic Church of my youth.
This was not, at first, a welcome discovery. But in time I asked myself: if Christ intended to found a Church, which He did (“on this rock I will found my church”), and if the Holy Spirit preserves that Church in existence, as He does (“the gates of hell shall not prevail against it”), which Church would it be? Where is unity? Where is continuity of teaching? Where are the heirs of the apostles? Not (with apologies to my Protestant friends) in the Protestant denominations, which are ever splitting; not in Eastern or Oriental Orthodoxy, which have the sacraments but not unity; but in the Roman Catholic Church. Here I am.