The difference between trade publishing and self-publishing reminds me a lot of the difference between Hollywood movie releases and the old B-movie method.
Hollywood puts the movie in hundreds of cinemas, hopes for a big opening weekend, then forgets about it because the next weekend’s movie is coming soon. If the movie doesn’t do well in the first weekend it rapidly disappears, even though the movie might have been a success if released a week before so it didn’t conflict with some other entertainment venue which kept people out of the cinemas that weekend.
B-movie producers would release the movie slowly, judge the reaction, see if any changes had to be made, and try to build up an audience over time. Unless it was abysmally bad, they could probably make a profit, but it might take a few years.
In a similar way, the trade publisher throws a book out onto the book store shelves for a few weeks until they’re returned, sells them as remainders and forgets about it. A self-published author might make very little money on their first novel, make more after they rewrite the blurb, change the cover and fix any issues early readers complain about, and sell thousands of copies a few years down the line when they’ve released other books that brought in some dedicated fans.