The quoted Seventh Letter limits the argument to writing about philosophy, not writing about other things: "There neither is nor ever will be a treatise of mine on the subject. For it does not admit of exposition like other branches of knowledge; but after much converse about the matter itself and a life lived together, suddenly a light, as it were, is kindled in one soul by a flame that leaps to it from another ..."

There's also the objection to "putting it forth into a world of discord and uncomeliness". If I was being uncharitable I could say that he fears criticism, like he's saying the ideas will be ruined if taken outside of his Svengali-like reality distortion field. (Or more charitably, he wants a good faith environment to discuss them in, instead of exposing them to hostile trolls who just won't get it.)

(This is supposedly a letter written by Plato, but probably is more like fan fiction written by his contemporaries.)