A scholar may understand many questions without arranging them in a separate book of numbered chapters. Possessing knowledge and compiling a formal discipline are different things.
Religious understanding in early generations could be transmitted through teaching, companionship, narration and practice. As questions multiply, collecting the relevant arguments, distinguishing cases and fixing terminology becomes more useful.
As an analogy, a person may speak according to grammatical rules without writing a treatise on grammar. Likewise, a doctrine may be known before the subject has acquired a fully developed, specialized written form.