Why? Coming from people who say sola scriptura it makes not sense. Is the Bible lacking in something? Is it not perspicuous? Does is say we should write books about it (and profit off of it?!?)? Is the Bible not sufficient and proficient? Is it not, after all in your mind, the word of God? If so, why does it need someone else on top of it?
Your side preaches one thing and practices something completely different.
“Sola Scriptura” means that everything needed for salvation is contained in the Bible - it doesn’t mean that man is unable to talk about the contents of the Bible, verbally or in writing. No man can add anything of merit to the Scripture - that’s the point.