Writing has been considered magical for over a thousand years lol
There are entire alphabets that are thought to hold power, such as Hebrew, and are thus considered holy.
Writing is not, and has never been, a thought form. It is a concrete form of manifestation that connects desire (emotion) to consciousness (Mind) through the physical act.
However, you are correct, that folk magick generally does not use writing because, traditionally, it was the purview of the educated upper class, and thus kept from the riff raff that the Powers That Be wanted to keep ignorant. The power in writing has been well known for centuries and became the staple of the ceremonial magician, as compared to, say, a cunning woman, who used herbs and roots in her potions.
There are numerous instances of writing leading to the physical manifestation of what was written about. The novella Futility, for example, foreshadowed (manifested) the sinking of the Titanic. In that story, published 14 years before the Titanic was built, the ship was called the Titan, it was considered unsinkable, it carried only the minimum amount of lifeboats, its measurements were almost exactly those of the real life Titanic, and it hit an iceberg and sunk.
There are also examples of writing affecting, and sometimes even destroying, the lives of the writer. Leo Tolstoy and Ernest Hemingway are two that come to mind.
If you want to know how to use writing as a magical vehicle, besides petition spells, pick up Taylor Ellwood’s new book The Magick of Writing.