The problem here is that the games being manually optimized by the graphics card vendors can have pretty horrendous code. To the point that I remember reading a

The problem here is that the games being manually optimized by the graphics card vendors can have pretty horrendous code. To the point that I remember reading a post at some point about a large game not even calling glEnd after a glBegin. So these big companies get the graphics card companies to manually go through and hack together some specialized code (including the shader code that will replace the shader code written by the big game company) to fix up how the game works, and optimize it so it works well. Part of the reason for this is that it's so incredibly difficult to get to the "happy path" of a GPU when writing graphics code. (at least, with more complicated APIs. I think the newer things like DirectX 12 or Vulkan provide a much lower-level set of primitives to allow the developer to get to that happy path much easier, rather than doing 5 incantations in a magic circle to get there) The underlying state machine and code for a driver is incredibly complicated, and the only people who know how it really works and can view code at all levels of the software are the employees of the graphics card vendor. So they're the only ones who can truly squeeze every ounce out of the hardware/software.