Question as per title: why not?
I think the right question is why would it require high graphic quality? Elaborate your argument, a question like that without proper backing up argument isn’t going to lead anywhere in my opinion. There are little advantage from one quality to another higher, ground simulator battles needs a lot of improvements, whom none involves graphics to limit or make players as equal but in mechanics.
There are huge graphical problems relating to SIM in VR mode, I go more in depth in this: Extend Vehicle Render Distance Limit in Air Sim [EC]
forum post.