Because the bible teaches that the magisterium has an office assigned to it by Christ.
As for ‘all scripture is useful’, what scripture is he talking about?
If people decide what is and isn’t scripture, then the Word of God is lost, and that is exactly what has happened.
Everything written in the Bible is God’s word, Scripture. No one can decide what is and isn’t Scripture. It is God’s word, not ours. They can choose to believe it or not, but the fact that it is HIS word never changes. So some people decide to add to or take away from God’s word to form a NEW doctrine, something that ISN’T in Scripture. Like, for example, Mary being sinless, or Mary’s bodily assumption into Heaven, Mary being the Queen of the Universe, or a Pope, etc. Those are clearly not Scriptural, in God’s word. Which is exactly why the Word of God is lost. And doctrines of fallen man are used to form religious systems. People rejecting God’s word. THAT is exactly what has happened.