Because “sola Scriptura” does not provide a solid basis for reliably recognizing inspiration and canon. Without an interpretive authority instituted by Christ, only subjective judgment remains. The Church preserves and authenticates Scripture.
Cf. Mt 16:18–19; 1 Tim 3:15;
Augustine: “I would not believe the Gospel if the authority of the Catholic Church did not move me to do so.”
The Reformation approach claims that Scripture alone is the rule of faith and interprets itself. But:
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Who reliably tells us that Scripture is inspired? If the reason is “Scripture says so about itself,” that is circular reasoning. If one relies on an inner feeling or being moved, that is subjective and would equally legitimize other “holy books” in other cultures.
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Who interprets authoritatively? The diversity of private interpretations in recent centuries shows that the formula “Scriptura sui ipsius interpres” has produced practically no unity. Blessed John Henry Newman called it “unreasonable” to assume that a complex, multifaceted, partly obscure book would be given without a “permanent interpreter”—inspiration guarantees the truth of the text, but not automatically its correct interpretation (cf. 2 Pet 3:16).
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Scripture and Church belong together. Augustine confesses: “I would not believe the Gospel if the authority of the Catholic Church did not move me to do so.” By this he does not say that the Church is “above” Scripture, but that Christ himself has given his Church the authority to authentically confirm and interpret the handed-down revelation (Scripture and Tradition).
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Magisterial confirmation: Dei Verbum 10 (Second Vatican Council) teaches: The interpretation of revealed truth is entrusted to the Church, to the college of bishops with the Pope; the Church serves the Word of God, it does not stand above it.
Sources:
Scripture (EÜ): 1 Tim 3:15; 2 Pet 1:20–21; 2 Pet 3:16; John 16:13; Mt 16:18–19; Acts 15.
Fathers/Teachers: Augustine; Irenaeus (Adversus haereses on apostolic tradition); Jerome; Newman.
Magisterium: Dei Verbum 9–10; Vatican I (infallibility in defined cases).