Why didn't the Buddha write a book outlining his teaching?
The four nikayas do not mention &quot;writing&quot;. However they do mention &quot;reciting&quot; many times. One mention, in particular is quite interesting:

AN8.20:6.4: It’s impossible, mendicants, it can’t happen that a Realized One could recite the monastic code in an impure assembly.

In another sutta, the Buddha instructs:

DN29:17.1: So, Cunda, you should all come together and recite in concert, without disputing, those things I have taught you from my direct knowledge, comparing meaning with meaning and phrasing with phrasing, so that this spiritual path may last for a long time. That would be for the welfare and happiness of the people, for the benefit, welfare, and happiness of gods and humans.

What is notable about these and many other quotes is the amount of care mandated in transmitting the Dhamma. The transmission of the Dhamma goes beyond mere written duplication via Gutenberg printing press or mindless scribe. The transmission of the Dhamma requires transmission of pure understanding, with care taken to ensure the meaning and phrasing of what is spoken and heard. Specifically, what is required is:

AN2.126:1.3: The words of another and proper attention.

In my own experience, I have found that hearing, reciting and discussing the Dhamma to be far more powerful and effective than mere reading. I have found the eye to be fickle in its grasping as it flits from here to there seeing what it wants to see as it skims the written word. In contrast, hearing and reciting and discussing demand far more attentiveness. And although the Dhamma is available for reading, I myself study the Dhamma by listening, reciting and discussing. The difference in understanding is remarkable. I cannot say what the Buddha intended. All I can say is that reading is too superficial for me. Instead, I rely on hearing, memorizing and speaking the Dhamma in concert. And that oral tradition does indeed work for me. The oral tradition facilitates proper attention.