OpenGL is supported in Windows, through installable drivers. Recent (Vista+) versions also ship with GL1.4 support via a GL->D3D translation layer, for users whose driver vendors do not ship an actual GL driver.

The main reason ANGLE is preferred is that in general, on Windows GL drivers are dramatically inferior to D3D drivers. They are slower, less stable, and sometimes omit key features. D3D historically has a much better story for preventing shaders from hanging/crashing video drivers, and for recovering from failures that do occur (lost surfaces, driver TDR, etc) - something that GL has now in modern implementations, but didn't have consistently when WebGL was introduced.

Between the more widespread availability of good D3D drivers and the stability advantages, I suspect you might not see ANGLE vanish anytime soon - despite the fact that GL drivers for Windows keep improving.