It’s because they equate the early church with their own as if they somehow correlate which they clearly don’t. The Fathers don’t have unanimous consent on everything. They had varying opinions. This is why there is an appeal to the magisterium to make matters clear. However, the Romans don’t agree with the Orientals and Orthodox on a few substantial matters. They can’t all be true so essentially one would have to choose this supposed apostolic church on a whim. No. There must be a greater authority. The Bible. And before they say they gave us a Bible. The Catholic Bible wasn’t settled until the council of Trent officially as per Catholic sources. The Protestants were already around. The Scriptures aren’t a tradition of the church. The scriptures are inspired by God through the pen of men who were for the most part Jews and largely based on the teachings of the Old Testament scriptures. Throughout history there was little to no doubt on what books/epistles were included in the canon. Why? Because the word of God is recognized not decided.