Before the web, centralization meant you couldn't get published unless you made a one sided deal if you were lucky with a publisher. You would have to buy 2,000 copies yourself and try to sell that out of a car.

When amazon opened that up it allowed anyone to publish and now we have other smaller sites like smashworlds that offer similiar.

Things are now so much more open as a publisher.

On the selling side you had smaller book stores being pushed out by Chapters. Chapters bought up many book chains and pushed out many smaller shops with better prices.

When Amazon came in they were fighting against the big box stores and they won.

Things appear centralized with Amazon but you can sell your ebook from anywhere (from your site/app/newsletter) you couldn't do that before. Everyone wants easy Amazon sales.