If it were that simple that would actually be better.
To be clear there a lot of amazing stories which don’t get traditionally published at all let alone get huge book deals or become bestsellers. Many modern classics didn’t get large advances.
There are a lot of books that aren’t necessarily uniquely “amazing” which end up being bestsellers because of celebrity association and/ or the publisher has paid such a big advance that they push and push this book through aggressive marketing in order to earn it back. 50 shades of grey done exceedingly well and it wasn’t an amazing story to say the least, but it did do amazingly in the sales charts for various reasons.
Amazing story does not always equal commercial success nor is commercial success proof of an amazing story.
Regarding it being “worth the effort” - I think most writers/artists find intrinsic value in creating a piece of work they’re proud of, even if it doesn’t earn them a huge amount.
However, if you look at it solely in terms of financial reward yes majority aren’t getting paid well for the time they’ve spent on writing the book.
And then you get someone like Amy Schumer who was offered an advance by Simon & Schuster that was millions higher than the sales projection - bonkers!
Completely agree @OpizpuHeuvHiyo and I can’t stand the debates online about if book piracy is OK.