[papsosouid]

Because that would be a terrible analogy. Using linux as an example actually goes against your point. We do have "20 different unix variants". Just because you live in a linux bubble, doesn't mean there isn't a world outside of that bubble. Almost all software that runs on linux isn't using the linux api, they are using a gradually munged up collection of APIs from over the years, including posix, bsd and sysv APIs. That software works on on dozens of different systems. Software written to a common API that is implemented by dozens of different systems seems a lot more like an argument in favour of more rendering engines implementing a common API.