Because Catholic Doctrinal pronouncement is in line with the Tradition and the Scripture

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Lutero:
You miss the point with alarming regularity. It’s interesting that when a Protestant points to God’s word you call it an appeal 'to his own own personal interpretation…' but when a Roman Catholic appeals to the pope and councils you don’t portray those appeals as reflecting mere opinion of the Roman Catholic, but rather you presuppose that what is inferred by the Roman Catholic is as equally true as the doctrinal pronouncement.

Because the Doctrinal pronouncement is in line with the Tradition and the Scripture which it is explaining.

In other words, nothing is lost in the translation for the Roman Catholic.

That is correct. Because we don't translate the Word of God. We pass on the message of the Church.

And when it comes to the gospel, why there is perspicuity within Rome that cannot be found in Scripture is a curious thing, especially given that Rome was to have based her gospel upon Scripture.

The Catholic Church bases her Gospel upon the Word of Jesus Christ. It is upon this Word that the Scripture is based.

Catholics have an anchor for understanding the Word of God. It is the Teaching of the Magisterium throughout the ages.
And we have a key for understanding the New Testament. The Sacred Tradition which preceded the New Testament and upon which the New Testament was based.

Protestants have cast these out and rely upon their own misunderstanding of the Scripture to multiply error.