Amazon, BN, and Apple don't pay advances (...to the vast majority of authors. Maybe a few exceptions for big name exclusives).

Many authors write books, send them to their agent to sell, and then get paid up-front by a publisher, followed by an agreed royalty should the book sell more than a certain amount. This offers a certain amount of security, more so than self-publishing.

The more important point is that for many authors, the international markets are a big source of revenue. Self publishing in English is one thing - sorting out the translation is quite another.

I think it's important to not tar all publishers with the same brush. There are actually a growing number of independent publishers dedicated to the indie/digital space.

Old habits (and legacy establishments) die hard.