aaa.. what objects visible for miles around? You're not talking about IL2Cod, are you? because, actually in IL2CoD those objects you are mentioning are NOT there, unless you are within a mile or so from them. Go at 3k, and look down from above those huge radar antenas for example.. and.. you won't see them!
In AC you can climb atop of the highest building from a town, and see EVERYTHING around you, the whole damn town.
And about processing.. again you don't know what IL2 is really processing. Just load the map, and don't spawn any actor, spawn a plane, destroy it then just walk with the free camera, and write down your FPS.
Then do the same from your airplane, but without any cockpit (to not have the FPS influenced by rendering the cockpit), and write down FPS again. Spawn an AI in your flight, and fly with him, and again note the FPS with your flight mate in view, and not in view.
Then compare them all, and you might be surprised
A lot of the "complex" calculation of which you are talking about in IL2, are not actually there. many actually happen only when triggered (like when pressing the fire button, or when bullets are actually hitting something).
And don't mistake a big number of computations needed to be done with a complex computation system. those are different things.
I think you would understand better all of this if you try to find out a game engine development book, and try to read it.
As it is, you really don't know what's this about, not even the ones of you which are programmers, but never worked as game programmers.