Being a good reader is something a dc can do alone. Being a good creative writer is something that has to be taught. You do not wake up one morning and know how to do it! It is very time consuming to teach which is why there is not enough of a focus on it in schools.
Given the enormously heavy emphasis on what used to be called English and what is now for some reason called Literacy, on the school curriculum, it is surprising if creative writing isn't covered. Personally I think that unless one aspires to be a fiction author it has a limited use and spending some time teaching oracy, which has now all but vanished in both primary and secondary schools, would be far more beneficial to children and society as a whole.
As for a high reading age - some of these tests are just lists of words which children have to read out. This demonstrates nothing more than the ability to decode words, which given that children are phorce phed phonics (I could copyright that) from the age of about 6 months these days ought to be pretty straightforward by the time most of them get to 10.
In secondary school a decent reading age will ensure a child can access the curriculum fully; or rather, if a child is struggling with reading at the start of secondary school it will limit the ability to access the curriculum.