My only guess is they have a parallel skunkworks working on the same thing, but in a way that they can keep it closed-source - that this was a hedge they think they no longer need, and they are missing the forest for the trees on the benefits of cross-pollination and open source ethos to their business.
This brings back memories of late 90s / early 00s of Microsoft pushing hard their proprietary graphic libraries (DirectX) vs open standards (OpenGL).
Fast forward 25-years and even today, Microsoft still dominates in PC gaming as a result.
There's a bad track record of open standard for GPUs.
Even Apple themselves gave up on OpenGL and has their own proprietary offering (Metal).
Then there is the whole issue of extension spaghetti, and incompatibilities across OpenGL, OpenGL ES and WebGL, hardly possible to have portable code 1:1 everywhere, beyond toy examples.