[shmerl]

It's like saying that Apple shouldn't support JavaScript in the browser and should use AppleScript or anything else as long as it's incompatible (remember browser wars?). Why they should use it is quite straightforward - because it reduces duplication of work. Why they don't use it is clear as well - because they want to tax cross platform development with that extra burden. I.e. they use development tools as lock-in tactic. That's simply crooked.

Some history can also help. It all started from Mantle. Basically AMD grew tired of current APIs being behind the curve, and designed one from scratch that matches modern hardware approaches. Then Apple and MS used those ideas to push their lock-in off-shoots (they copied most of the ideas verbatim, working closely with AMD). Meanwhile, AMD simply opened up Mantle (they voiced this intention from the beginning) and gave it to Khronos as an initial proposal for Vulkan. The rest you probably know. So why exactly MS and Apple didn't get behind the shared effort? Because they are lock-in jerks. That's all there is to it.