Took him a while, though. Remember that he was originally employed by Microsoft and when he founded Valve and they licensed the Quake 1 engine, among the first things they did was to port the engine to Microsoft Direct3D (luckily they kept the OpenGL renderer because back then Direct3D ran worse).
Same reason GabeN switched.
GabeN actually used to work for Microsoft, untill he realized more people had DOOM installed on home PCs, than… Windows.
He helped basically port DOOM to Windows 95 / DirectX, realized you can run games on PCs and there was a market for it, therefore, found Valve.
Took him a while, though. Remember that he was originally employed by Microsoft and when he founded Valve and they licensed the Quake 1 engine, among the first things they did was to port the engine to Microsoft Direct3D (luckily they kept the OpenGL renderer because back then Direct3D ran worse).
It took them both quite a while but at least they seem to be coming around.