Amazon does have a cloud SaaS office suite, actually -- Amazon WorkDocs / WorkMail / WorkSpaces. I've hardly ever heard of businesses using them, though; Office 365 and Google Apps have the market pretty well locked down.

I feel like there is a fundamental distinction to be made between infrastructure services (like AWS S3, EC2, RDB, etc) and end-user services (like Office 365). Combining the two doesn't make a lot of sense; procurement for the two typically happens separately, and businesses frequently mix-and-match vendors for infrastructure and end-user services.