The fact is Microsoft forced ATI and NVidia to remove hardware OpenGL support from the drivers that ship with XP while spending significant resources ensuring all graphics cards include certified DX support.
The DirectX drivers supplied with XP don't work properly either. I had NVidia GeForce 3 cards crashing left, right and center with the default drivers, with DirectX apps. I had to download and install NVidia's own drivers to get them to work.
The vast majority of PC's out there are store-brought. As I mentioned before, they should have the proper drivers from the manufacturer since any ones shipped with the OS will be both sub-par and incompatible with newer cards.
Edit: I agree that there is a danger of OpenGL getting left behind DirectX in terms of features. But lets face it though, hardly anyone here could make use of such features. Only a relatively small number of developers have taken Blitz3D to its limits.