Valve specifically tailors their games to the low end of the hardware market, similar to Blizzard. The HL2 engine is DX9, not 10, and runs great on a huge range of hardware... including and most specifically store-bought PCs with stock videocards and processors.
If I recall correctly, there are only two games on the market designed for the PhysX PPU (both of which have GRAW in their title). Not to mention that the Half-Life 2 engine specifically uses the Havok SDK as opposed to the PhysX SDK.
There might be a mac gaming market (I sincerely doubt it) to promote, but don't generalize about the established PC market if you don't know what you're talking about.