Okay, there are the outliers, but writing a best seller on the Stephen King/JKRowling/EL James level is as normal as winning the lottery, but less frequent.
Basically, writing a best-selling book, when you calculate the hours you spent, usually works out at a bit better than minimum wage. You just tend to get the money in a lump rather than week by week. Don’t forget, you have to calculate not only the hours you spent writing, but also the hours researching, rewriting, editing, reading galleys, promoting it, doing interviews, doing tax returns, maintaining your social media presence as well as the actual writing hours.
The trouble is, just writing one best-seller is not enough. That’s a nice chunk of change, but you can’t live on it. You have to keep writing books that sell. Anything that bollixes that, whether it’s a change in writing fashions, or an illness, or a family disaster, mean that you don’t have a new book to bring in more money.
The majority of professional published authors also have a day job.