Because the authority of Scripture does not function as a watershed unless it becomes effective in creating a heart of glad agreement, and a mind of transformed perception. It is possible to say that the Bible has authority (and sign an affirmation of faith), and yet not see as real what the Bible says is real, and not feel as precious what the Bible says is beautiful. Until we regard as real what the Bible regards as real, and until we rejoice in what the Bible rejoices in, its authority may be affirmed, but it is not effective — and it is not a watershed.
For example, the Bible says of Christians, “You have died” (Colossians 3:3). And, “You have been raised” (Colossians 3:1). And, “Your life is hidden with Christ in God” (Colossians 3:3). Those are realities. But thousands of Christians have nothing in their minds that corresponds to those realities. If you asked them to point to realities in their lives that correspond to these things, they would not be able to. Therefore, these biblical teachings are not effectively authoritative in their minds.
The same thing can be seen in regard to what the Bible says is valuable or beautiful. Philippians 3:8 and Matthew 13:44 teach that Jesus is more valuable than everything we own or could possess in this world. But thousands of professing Christians value other things more than Christ. They rarely talk about Christ as pleasing. But movies and social media and sports and politics fill their animated thoughts and conversations.
This is because the revelation of the supreme beauty and value of Jesus is, for them, not effectively authoritative. The authority of Scripture is affirmed the way wrapping paper is affirmed: “I love this gift. It is beautiful” — meaning the wrapping paper is beautiful, though the contents are unknown, displeasing, or simply negligible.