They wouldn’t, simply because their position as the dominant player in ePublishing gives them a massive incentive to lock-in users. Which is what their DRM does: you cannot move books to non-Amazon devices, so you’re stuck on Kindle forever. That this also satisfies publishers is something of a side-effect.
They did the opposite with music (they actually sold simple standard mp3s, unlike what you get from iTunes) because they were challengers, not incumbents, so the priority was to attract users.
