This is an odd place to put a stake in the ground--there are a number of macro trends that have been going on for far longer (e.g. the military-industrial complex, the Cold War, Congress, American football), as well as a few others that have only really come to a head more recently (e.g. demographics, media spheres/tribalization). I would argue that our failure to learn lessons from the Millennium Challenge has a massive overlap with our failure to learn from Ukraine--not to mention Iraq, Afghanistan, Vietnam... The military is not monolithic--remember that the Millennium Challenge was more or less a sparring bout between two parts of the military with different philosophies--and it really takes something like an existential war for meritocracy and common sense to reassert themselves to a meaningful degree.

A smaller point: all military exercises are heavily scripted--it's more or less impossible for them to be otherwise, as you just can't simulate the details of war that matter without actually killing people, breaking things, and giving up your secret game plans. Usually the goal of this sort of thing is to make sure that everything (people, equipment, doctrine) works together more or less as intended, and people have the experience leading and operating in larger units than they do on a routine basis. The PR people then spin it into an unqualified and historic success, validation of our technology and tactics against the forces of evil, blah blah blah. It is still very difficult to draw the right lessons from these sorts of things--even more so when the civilian leadership of the military has 99 things to consider besides a certain kind of pure military effectiveness (and although I have strong feelings here, we're still doing quite well on the tactical and operational levels in spite of everything).

Fun fact: the Millennium Challenge is still taught as a case study in basic officer training, at least in the Marine Corps (well, probably--it definitely was a little over a decade ago).