It is not as simple as "let everyone select the difficulty they want".
1.
There is a portion of the market that wants to be able to boast about "beating a game on the hardest settings" without actually putting too much effort in. It's considerations like that that which has led to the "dumbing down" of quite a few games.
Examples include Gothic III where a very well thought out combat system in Gothic II and Gothic I was replaced with US style "stand next to the monster and mouse click as fast as possible", the more recent versions of the Warhammer tabletop game franchise and of course the classic dumbing down of D&D in the recent 4th edition.
2.
The issue arises of how many people will use a feature versus the work involved implementing it.
If for example the work involved in realistically simulating the effect tire pressures have on takeoff means losing an entire extra aircraft and only a handful of users will ever check those tire pressures before takeoff, the feature is best left off, at least initially.
PERSONALLY I am interested in as much "realism" and "immersion" as possible and for example in Il2_1946 fly with cockpit on and LHS speed-bar and RHS HUD turned off
However there is a limit to what can be implemented in a mass marketed game.