grammar and rules of language has almost nothing to do with the difficulty of writing good text (ala, written prose fit for purpose).

Understanding of grammar and rules of a language is like having good bricks, but building a stylish house is more than just using good bricks. It's a higher level cognitive exercise. Schools attempt to teach this, but i think they fail quite miserably (look at how essays had to be written by students, and how poorly most are written).