Richard-
I think you may be asking two different questions. One has to do with why God doesn’t speak to us in an unmistakeable way. I think this gets to the issue of faith and love - that God does not want to just “make us” love Him and follow Him, because that wouldn’t be love, it would be coerced. God gives us free will because He has made us in his image, and that includes being free.
As to your specifically stated question, I disagree that it is easy to write language that doesn’t need to be interpreted to be understood. Virtually every written communication is subject to varying understandings, as it is filtered through the mind of the reader, each of whom brings to the process their own experiences and understandings, the ways they use language, etc. Written communication loses the impact of vocal inflection, body language, and the opportunity to restate and reshape in response to the responses of the listener. Just think of how often we see misunderstandings in emails for this very reason.
Now add to the inherent weakness of written communication the implications of passing down written communication through generations and in multiple, ever-changing languages. Think about the difference in the ways people thought about and understood the cosmos in Moses’ day and the ways we think about it now. What examples, what references, what images could be used that would have exactly the same meaning for us that they had for them? Consider even the ways English has changed in the few centuries since the D-R and KJV translations were done - (Luke 18:16 - “Suffer the little children to come unto me” Huh? “Suffer” has changed meaning drastically in the intervening years.) Even documents written with every effort to be explicit, clear and unambiguous frequently require re-interpretation over the years (think of laws in general, or the U.S. Constitution in particular.)
Then, finally, there is the question of inspiration. We do not, obviously, believe that God wrote the words with a giant quill (although many of us probably thought that when we were children). We also do not believe that God dictated the words that were written. The process of inspiration includes what was recorded, and how and by whom, and what was preserved, and how and by whom, and what was determined to have authority, and how and by whom. So the Scripture we treasure today includes fragments of ancient songs, multiple narrative accounts of the same events, a variety of genres and themes, all passed down, transcribed, copied, translated and copied again. It is, in some ways, a process more astounding than it would have been had God written it with a giant quill! But it is, as you rightly observe, not straightforward.
But insulting? I don’t think so. If you think someone is an imbecile, you speak slowly and directly and try not to leave any possibility for interpretation and misunderstanding. If you are engaging in conversation with someone you respect and love, your language is very different, more interesting, more inspiring, more poetic, more playful.
I don’t know if this helps answer your query at all, but it’s an interesting thought process- thanks for that!