It is very likely because some of the new graphics features are supposed to be requiring DX11 but who knows… maybe Bethesda kind of 'grafted' the effects on top of the DX9 engine with workarounds. It has not been 100% confirmed to this day AFAIK that the renderer has been upgraded to DirectX 11. Windows 10 is always 4GB only if/when DirectX 9 is involved. An upgrade to a 64-bit exe would only benefit Windows 7 users if the engine is still DirectX 9 because Windows 10 simply can not use more than 4GB in DirectX 9 games regardless of whether you have a 32-bit, 64-bit or hypothetical 512-bit exe. And the impact that can have with memory and mods.
The draw for PC players is the 64 bit exe. The graphics improvements are for console gamers not PC. I'm surprised many here are complaining about graphics.
