Presumably they don't run realistic exercises all the time - that sounds like a military equivalent of always being in crunch mode and we all know how well that works out!
Wartime can last years, you can't rely on only being able to withstand that for a couple of weeks or months.
I can't say I have any experience with the military, but from listening to "Life and Work on HMS Enterprise"[1], they are constantly running realistic exercises (in their case, mostly simulating fires and people overboard, since it's a research vessel).
