erg, the restrictions are due to 1) human limitations and 2) structural limitations.
In the case of the "new" planes of AH, 2) is represented, but 1) is not. In the case of the "old" planes, 1) and 2) are not both present(look at the Bf109G-10 for instance.. true, that the Galland Hood helped better the visibility..but it's too good. With head positions maxed out, you virtually get a one-look panoramic six view which covers 5 to 7o'c position in one touch!).
In the case of WWIIOL, or IL-2, there are restrictions to both 1) and 2), but both excessive.
What I am picturing is restrictions on both 1) and 2), but not as excessive as IL-2 or WW2OL. As you can see in the posted picture of the P-40B cockpit, that's the 'reasonable' level I figure. You will notice it is possible to check your six, but you will have to do that separately for the left half of your six, and the right half.
In the case of the P-40 in IL-2:FB, you get only about half of the visibility shown here in the pic. You have no 6 view at all, and it only a very very narrow 'glimpse' is possible by viewing your 5 and 7 oc - this, is not the case I'm suggesting.