Most of it is Nvidia, The Graphics Mafia, wanting their products to excel in a way that'll make them more money. Your OS and your game developer can only optimize so much.
If you haven't followed performance benchmarks of various OS versions over the years, I guess you missed that OS optimizations can make a significant difference. Especially if the way the display is linked to the kernel is radically different.
Almost all operating systems use a certain approach to the display model and window server/manager, based around the ancient X Windows concept in some fashion or another.
Yes, many variations exist, Android uses a custom window manager, Wayland is a new competitor, and there are goals to improve X to allow direct framebuffer access and GUI presentation without interacting with the display server.
OS X and iOS basically draw frames as PDF in the framebuffer, as a compromise to the concept of Display PostScript used in NeXTSTEP, which helps significantly with print matching and other rendering functions... reasons why Apple (and NeXT before it) is a major player in the art and printing markets.
Windows, I think, has the most efficient system when it comes to games. There are advantages for the other systems, but I think there are performance penalties. I don't think there is anything resembling a display server within Windows, likely a product of Windows being so radically different from Unix-like systems.
Plus the other reasons I mentioned.
Nvidia may play a heavy hand, but all they want is their products to look good. They make Linux and Mac drivers. And film rendering and CUDA/OpenCL coding on such platforms is a major source of revenue (through the professional GPU market).