That middleware is a different beast though, they are different engines providing different solutions for different problems - you can't have any single one of them be the solution to all problem and most of them are very complicated to be defined as a standard that is supposed to live for decades.
OpenGL, Vulkan and Direct3D are at a level where they enable you to write your own engine, but not at the level where they provide your the engine themselves. Microsoft tried it with Direct3D RM and it didn't work and Sun also tried it with Java3D as the official way to do 3D in Java but also didn't catch on (it caught on more than D3DRM but that is mainly because there was no other official way - however its popularity paled in comparison to OpenGL bindings that appeared soon after and nowadays it has been reimplemented and lives on top of these bindings).
OpenGL, Vulkan and Direct3D are at a level where they enable you to write your own engine, but not at the level where they provide your the engine themselves. Microsoft tried it with Direct3D RM and it didn't work and Sun also tried it with Java3D as the official way to do 3D in Java but also didn't catch on (it caught on more than D3DRM but that is mainly because there was no other official way - however its popularity paled in comparison to OpenGL bindings that appeared soon after and nowadays it has been reimplemented and lives on top of these bindings).