In order to cut traditional publishers out of the loop, they need to have a critical mass of Kindle users.
What do you think is going to happen when Amazon announces that they'd be happy to give any author 25% of all sales if they publish direct through Amazon as opposed to the 5% their publishing house gives them? All the traditional publishers will immediately pull their properties off of Amazon to try to kill their new rival (or at least, try not to keep feeding the hand that is strangling them). So Amazon's Kindle readership has to be big enough that the readership stays with them when a huge amount of back-catalogue suddenly stops being available for purchase, big enough that authors will leave behind editors they have developed working relationships with over decades to have access to.
Amazon isn't there yet.