It might be that only a small percentage of writers are international bestselling authors because a) only a certain percentage have the pre-existing fame that guarantees sales; b) only a certain percentage write the story that hits the market at just the right time; c) only a certain percentage write the type of story that appeals to a wider reading audience.
As for people who are good writers, there may be a relatively small number compared to those who say they want to be writers because wanting to be something and becoming that thing are different. I might want to be a world class potter, but would I still want to be a world class potter if it meant putting years of my life into learning the art of all the things that go into potting? You have to be pretty passionate and dedicated to get very good at anything. If you don't love it, you won't put in the time and work, and you won't get good.
"Being able to come up with creative ideas isn’t something you need an overly-high IQ to accomplish. Once you’ve got a level of knowledge gathering and utilization that’s about average, you’re well on your way to having the creative potential of Albert Einstein, Bill Gates, or Steve Jobs." https://creativesomething.net/post/41103661291/the-relationship-between-creativity-and