I'd say to have a common interface towards hardware that is quickly becoming similar across smartphones, tablets, and even laptops (read Chromebooks)?

I think why they would _not_ have the same interface between similar hardware and similar applications is a better question.

As for the user interface and bundled applications, let's not confuse "Operating System" with that although it's popular for some stupid reason. The one and same OS could of course have e.g. completely different window managers adapted for different human-device interactions and use cases. But that's very distanced from the actual OS, that is pretty much only interested in how to run and expose the hardware to the software.

Save man-centuries of platform-specific work?