You could make similar arguments about electricity or water. Similarly Medium or YouTube. Why allow someone else to host your videos or writing?
Components higher and higher up the value-chain get commoditized and become utilities.
There’s always a risk of monopolistic behaviour — hello Comcast, please can I get HBO Go on my TiVo? — but Amazon and their ilk must know that such behaviour will make space for disrupters, and there’ll always be several likely candidates waiting in the wings.