There are at least two reasons for this.
(1) The Bible itself is full of creeds, confessions, and catechism!
The following passages prove that it is unbiblical to say “No creeds! Or confessions! Or catechism!”
Creeds and Confessions:
1 Corinthians 8:6
1 Corinthians 12:3
1 Corinthians 15:3—7
1 Timothy 3:16
1 John 4:2
(2) The apostles commanded their disciples to preserve the essentials of the Faith for future generations of the church, and they intended this to happen by means of creeds, confessions, and catechism.
Consider these words from the apostle Paul to his beloved disciple Timothy.
Follow the pattern of the sound words that you have heard from me, in the faith and love that are in Christ Jesus. By the Holy Spirit who dwells within us, guard the good deposit entrusted to you (2 Timothy 1:13—14 ESV).
The “pattern of sound words” is earlier called “sound doctrine” by Paul.
“Sound doctrine, in accordance with the gospel of the glory of the blessed God with which I have been entrusted” (1 Timothy 1:10c-11 ESV).
These “sound words” or “sound doctrine” do not point directly to the Bible, but rather to summaries and core doctrines that express what the Bible teaches on the most important facts of the gospel. These are what early church fathers, like Clement, Irenaeus and Tertullian later called the Regulae Fides, that is the Latin for the “Rule of Faith.” This referred to the summary of the non-negotiable key doctrines of the Christian faith, which have been believed and expressed by all Christians in the Apostles’ Creed for nearly 1700 years.
The apostles, then, commanded the core commitments of the Faith, as faithfully deduced from Scripture, according to the apostles’ understanding of the meaning and purpose of Father’s plan in sending Jesus Christ into the world, whose work is then applied to the church, individually and corporately, through the power and presence of the Holy Spirit.
Therefore, “No book but the Bible” is actually against the teaching of the Bible!