Because WebGL is an evolutionary dead end, for a variety of reasons. The initial idea was to track OpenGL ES, but that isn't really true anymore. Because of Windows, WebGL has to stick to a subset that can be easily translated to Direct3D. Because of GPU process sandboxing, anything that flows back from GPU to CPU is a huge synchronization problem. On top of that, mobile GPU drivers continue to suck badly, which means most of the extensions that were supposed to expand WebGL's scope into this decade are still out of reach, with 50% or less support in practice.
On the flipside, the native low-level APIs have all diverged. Vulkan, D3D12 and Metal each made different decisions, so it's pretty much inevitable that a 4th standard will have to be created to unify them. It will probably be higher level than any of the 3, and it will still be subject to strong sandboxing limitations.
Personally I think the big issue is that people stare themselves blind on the traditional graphics pipeline. Modern renderers have evolved past this, with various compute-driven and/or tiled approaches common place now. They're a nightmare to implement against current APIs, because a small change in strategy requires a rewrite of much of the orchestration code. The job of figuring out how to map your desired pipeline onto the hardware's capabilities should be the job of a compiler, but instead people still do it by hand. Plus, for GPU compute-driven graphics to be useful for interactive purposes beyond just looking pretty (i.e. actual direct manipulation), you absolutely need to be able to read back structured data efficiently. It's not just about RGB pixels.
There's an immense amount of potential locked inside, but the programming model is a decade or two out of date. Only AAA game companies and the vendors themselves have the resources to do novel work under these constraints. Everyone else has to throw together the scraps. Even the various attempts at LISPy GPU composition fall short, because they don't attempt to transcend the existing pipeline.