Agreed! Making all your API calls to Kloudless (or cloud elements, cloudrail, etc...) instead of individual services is certainly nicer for the developer, but creates another for-profit entity that's just in the middle of it all.
To really be like email we'd be talking about a common protocol that all the service providers implement so software can speak to all providers directly in the same way without a middleman. (A better WebDAV with industry support?)
Well, there have been attempts to create single standards (e.g. CMIS, in the content management space). A lot of the time, enabling the right workflow for the integration to be useful involves providing capabilities that quickly diverge from any common standard that is agreed on. Tying together different business stacks involves a lot more than just establishing API endpoints for individual actions.
Having a single API reduces this inevitable maintenance overhead and swaps out some set of N integrations in favor of one other. There is the added benefit of having a resource (Kloudless, in this case) assist with enabling whichever use case the business is trying to solve for their users by providing the integrations.