Part of the reason why aircraft like to hunt in mountains on the 104th is that we have much less situational awareness then we would in real life.
No AWAC/GCI, or datalinking that would be available in real life due to their buggy nature which means that all the information we get is from the 60 degree arc from our radar and almost blind 1.5-2nm visual detection, so you stay in the mountains to have more chance of spotting visual threats, especially since shadows are much easier to spot then actual aircraft.
And what's the point of staying at 35,000ft when missiles are broken and you don't get a significant range advantage over a someone at 5,000ft when the missiles don't loft and under 20,000ft they deploy a parachute.