They have invested millions into creating cloud software. If they open source it, what is stopping other firms from taking that software and creating their own cloud offering?
Would you work for free? Would you open source all the work you have done?
(Also, yes, I open source as much of my work as possible, and I'm paid to write it)
Also, AWS is BUILT on open source, they just don't open source the extra bits they added on top. Just like how Microsoft took an open source tcp/ip stack and open source web browser, added some stuff, and called it their own.
Why would they open source it? To provide a way out to customers. They don't even provide a mongo-like license.
I can't forget a session at Google I/O how they were insisting that appengine is not open source because we are too dumb to understand it....yeah sure what about k8, now? I can run it on my Mac, I even contributed few commits.