They probably could, but it isn't happening. It is hard to find a cloud service that seamlessly works with some other cloud service without explicit integration

They probably could, but it isn't happening. It is hard to find a cloud service that seamlessly works with some other cloud service without explicit integration between the two services. Email was introduced and has become a popular standard long before commercial cloud services. The companies adopted it, but they do not seem to be interested in introducing similar standard protocols for new APIs.