>Isn't it extraordinary that these models still can't write well?

It's not at all extraordinary if you understand how they're trained. For math and programming it's possible to generate an infinite amount of training data with correct answers, as compilers and theorem provers can guarantee correctness. For writing, the quality is not objective, it's determined by human preference, so you need humans in the loop, which slows things down massively and limits the amount of data that can be generated.