Some people just find writing natural to them others have to struggle. Asimov was a famous natural writer and cranked out 430 plus books ! Not all of them are sci-fi or fiction either.
That is basically averaging 10 books a year for 43 years to get to that kind numbers . He probably wrote more during his peak years
He had to do it all of it on a typewriter, it takes more effort both physically and also mentally as there is no backspace you have think more carefully or you would have lot of corrections and retyping pages.