They need a new more developer-friendly API built on top of Vulkan as a replacement for OpenGL.
If you care about performance it will just do something slow at uncontrollable points in the background, like copy CPU-GPU-CPU memory, synchronize against the GPU, do format conversions, etc.
If you're the one implementing GL, it's gotten gigantic again since they simplified it. GL 4.3/4.6 core has compute shaders, which means you have to implement OpenCL twice but different this time.
It also doesn't come installed on my machine.
OpenGL also does not come installed on my machine.