So how does that help "discoverability"? Amazon is able to recommend books based on the buying habits of others with similar taste as those with my buying history. They are able to list the top sellers, etc - none of which would be available on 8 different websites.
Then that also means that you have to sign up for 8 different websites and give your payment information to 8 different websites. Do you really think this would be more convenient than going to one website, buy an ebook and then it automatically shows up on all of your devices?
I can start reading a book on my iPhone, go home and pick up my Nexus tablet and everything is automatically synced -- bookmarks, notes, the page I was on etc.
If I buy a new device, I can install the Kindle app and my entire library is automatically there with all of the meta data no matter which device.
How would you propose you do that with random ePub readers?