This is just my opinion, but I think that an API, like you seem to be talking about, would make sense for something running an operating system that has libraries and/or device drivers. One or both of those would provide a level of indirection necessary to interface a standard API with devices by various vendors.
With little 8-bit chips like we normally talk about here, the chip is all there is, so it has to have knowledge of the vendor's API to the device. Unless we could get the various manufacturers to all use compatible interface protocols, we're left with what we have.
Edit: You could provide the equivalent to device drivers or libraries with additional chips. If you program a chip to know how to talk to a particular device, then provide an API to it (with I2C or whatever), you could control multiple devices from various vendors with one API. That's kind of the same thing as stuff like this
. It may or may not be more expensive than desired, or take up more board space than desired, but that's one way to go.