That would be because the Catholic, as well as the Orthodox, realizes the Holy Bible is only a very small part of the Depository of the Faith!
In fact, they also know the Bible passages of how they must eat of the Flesh and drink of the Blood to be saved (John 6:53-55) (Sacrament of the Holy Eucharist) and of how everything Jesus taught could not be written down (John 21-25) but only taught through the Apostolic Succession (Bishops of the Catholic Church); this is the Sacred Deposit of the Faith, Catholic Tradition.
Protestants have not only lost all of this but they have also lost a great deal of the Holy Bible as Martin Luther decided he could not only change the wording of the Bible but also just cut various chapters and books out that he did not agree with, personally. This is why the Catholic and Orthodox Bible has the full 73 books.
So, yes, it may be hard for some Catholics as they realize they cannot interpret the Holy Bible on their own – it would be folly to think 40,000+ different interpretations of the same thing could be anything but separating the wheat from the chaff which is why the office of the Pope was created by Jesus, Himself (Luke 3:17, Matthew 16:18 and Luke 22:31).
As for Peter's authority, there is ample evidence Peter was first in authority among the apostles. Whenever they were named, Peter headed the list (Matt. 10:1-4, Mark 3:16-19, Luke 6:14-16, Acts 1:13); sometimes the apostles were referred to as "Peter and those who were with him" (Luke 9:32). Peter was the one who generally spoke for the apostles (Matt. 18:21, Mark 8:29, Luke 12:41, John 6:68-69), and he figured in many of the most dramatic scenes (Matt. 14:28-32, Matt. 17:24-27, Mark 10:23-28). On Pentecost it was Peter who first preached to the crowds (Acts 2:14-40), and he worked the first healing in the Church age (Acts 3:6-7). It is Peter's faith that will strengthen his brethren (Luke 22:32) and Peter is given Christ's flock to shepherd (John 21:17). An angel was sent to announce the resurrection to Peter (Mark 16:7), and the risen Christ first appeared to Peter (Luke 24:34). He headed the meeting that elected Matthias to replace Judas (Acts 1:13-26), and he received the first converts (Acts 2:41). He inflicted the first punishment (Acts 5:1-11), and excommunicated the first heretic (Acts 8:18-23). He led the first council in Jerusalem (Acts 15), and announced the first dogmatic decision (Acts 15:7-11). It was to Peter that the revelation came that Gentiles were to be baptized and accepted as Christians (Acts 10:46-48).
But without this fundamental understanding to begin with, how then can one go on to embrace the full Sacred Deposit of what has been revealed, let alone all 7 Sacraments required for Salvation.
Although, remembering the difference between the big rock (Jesus) and the little rock (the Pope) helps in beginning to understand. Not to mention the fact Jesus spoke in Aramaic, not Greek. So, understanding "kepha" is crucial as well as comprehension that in first century Greek the words petros and petra were synonyms. So, full, accurate knowledge of history is crucial.
