Just because one can speak English well doesn’t mean that one can write English well too. Even some native speakers studying in colleges and universities in English-speaking nations can’t write an essay of 5 or 6 paragraphs long in English clearly and correctly. They have all the opportunity to practise English in the four skills in their environment, and yet they can’t express themselves in the written word. Some of them. Not all of them.
More evidence needs to be provided on the success rate in various environments that the spoken English that students hear in class and in stories is reflected in their writing.
If that is the case, why can’t some native speakers in English-speaking nations write English well when they hear it spoken in class and in their environment as their environment uses only English?