There are reasons for this. Beyond Korea (and probably Korea as well) would simply be a fratricide of American hardware in an AH style game or a very lopsided game if it was done NATO vs Warsaw Pact. In addition the modeling of modern avionics and missiles is far more time consuming. Beyond even that is the complete lack of fun of BVR combat.
Oh, I disagree with this. I really do.
If you do the 50's, then I think a F-100 saber vs a Mig-17 isn't that cut and dried who is the better fighter. Going further with the "century series" of American planes the F-102 was a poor performer even at the time. The 104 Starfighter was a good fighter, but had no radar, and the F105 is a light bomber. The F-106 was just an improved F-102, but again it is an Interceptor built for speed and radar guided missiles, and not at all good in a dog fight. I should not that the F-102 doesn't even have a gun.
Match that up with Mysteres, Hawker Hunters, Saab Draken, Mirage III, MiG-17, MiG-21, and the Sukois of the time, I don't think you will see that the USAF is going to dominate. The USN had the F-3 Demon, F-4 Skyray, F-8 Crusader, and the F-11 Tiger.
Not many of these plane I would consider superior out of hand. I actually don't know which planes would be the "best" planes of that era, but I want to find out.
As for the "lack of fun in nothing but BVR combat" I would point out that I don't think that is the case at all. I've played Strike Fighters and Wings over Vietnam quite a bit, and you spend a LOT of time in dogfighting mode at close range. Given that poor performance of missiles of the day, the most common missile kill is with a heat seeker, and they only work if you are behind the other guy.