Being an author is only partly about being a writer. Lots of people who write well, don’t necessarily write well across-the-board. Someone proficient in government report writing may have style-guide perfect grammar but might struggle to write a compelling science fiction novel. An advertising copywriter may have the gift of expression but could struggle to convert that in literary terms.
Yet both are paid to write.
Being able to write is only half the job. What remains is craft. Whether you’re creative creationist (you think of writing as a ‘gift’ more than a ‘craft’) or an evolutionist (you think the skills of writing can be learned, like any trade), constructing a saleable product can best be learned through experience, trial and error or lots and lots of research.
Think of it like a professional swimmer suddenly deciding to take up pro cycling. By virtue of already being a sports professional they do have an edge–fitness, discipline, competitive streak–but that’s not all it takes to make a pro cyclist. That swimmer still has to go back to the basics of the cycling discipline and learn the craft. They still have to practice until proficient. Then they have to get better than every other cyclist out there if they want to win races.
Just like in sports, there will be those writers who make the transition to author with less training than others. But before you start making voodoo dolls of those lucky people, keep in mind that even they have to then re-train again from scratch to transition from author to published author.
A whole different discipline in a massively competitive arena.