One significant advantage of AWS is that they have a very large, built-in retail customer with relatively demanding requirements. I've always been under the impression that Microsoft doesn't really have a business that would lend itself to consuming its own services[0] and it's unclear to me whether or not Google uses their stuff internally.

0 - as an anecdote, I recall their salesmen coming to sell my employer their caching solution around 2008. I asked the rep I'd Microsoft was using their caching product on any of Microsofts web properties and he replied that they were not.