If you're good at it, which extremely few people are. It's my belief that not enough good writing exists for the tech companies to train an LLM. At least not a generally applicable on.
Tech companies are training off the shadow libraries, which have the bulk of English-language human writing in book form from the twentieth century, and that traditionally published stuff with an ISBN usually got a proper editor of some kind. So, it is hard to believe LLM companies don't have enough material; it's not as if they are limited to low-quality stuff on the web.
It might come down to that they don't care (enough). They could probably do it if they had to (hire a lot of professional editors and authors to train models), but AGI seems more profitable and also a better "story" for investors than models that are good at writing.
Thing is, LLMs don't seem to be the right vehicle for AGI, but even if the nerds understand that, see above about profit perspectives and investors. These companies are under insane pressure to deliver the next super hit or be seen trying.
