They would need to re-render the world to put mirrors in. Not going to happen any time soon, even for extraordinarily large values of "soon".
Given the limited utility of a mirror as opposed to looking backwards (and nobody in an AH aircraft is old, stiff, or beat to crap and can crane their necks around to angles that would normally break important bones...trust me on this) it's far better to set your views. Play with head position until you get the best view possible for the aircraft you are in, then save the head position. Your view out the back of an F4U-1, for example, is only slightly less sucky that the view out the back of a B24, but it gets the job done. You can actually get better vis than might be possible with the actual aircraft tightly strapped to your back, but that pretty much makes up for the monitor being a slightly less realistic view than, say, the Mk 0 Eyeball. It's all a trade off.