One of the things that kind of pushes publishing along, at least traditional publishing, is international rights. Each country is a different market, and when a writer publishes a book it is typically in just one country. Each country is negotiated separately, and for many writers those foreign markets are where they make a lot of their money.
If the book is doing well in the US the writer will have a chance to negotiate a separate deal in say the UK. That is one of the easiest deals, because both markets are English language, and you probably don’t bother changing the spelling of all the words that don’t match. That UK deal will be a much better deal for the size of the market, because the book already has a track record. Even if the book has a pretty mediocre track record you can still make money by going to a bunch of different markets.
Now, with self publishing, you have a global market. A single international market that consists of every country on earth. Amazon pushes books out to the entire globe. Of course you already have a better deal for royalties than you will ever get from traditional publishing, so it’s good for you there as well.