The US fails to learn the lessons of it's last... idk, every war since WWII. Leadership cites kill stats when it has nothing else to cite, but killing, while certainly an important part of war, is not the key to victory and that's why we keep losing. Actually winning wars requires controlling strategic points on the battlefield, both literally with armed combatants, and metaphorically by getting civilians to support you. The US fails at both. We show up with incredibly superior firepower, establish FOBs where we think we need, drive armored convoys to supply those FOBs but they take fire on every trip, have to look for and clear IEDs, what have you. There's no control apart from the FOBs themselves, and the enemy has easy run of all territory not being patrolled at that precise moment.

This is the same problem that doomed us in Vietnam, in Korea, in Iraq, and will doom us in Iran. It's also the same problem that fucked over South Africa and Rhodesia and seems to be a common problem for white supremacists, but that's just my editorializing.

In any case, showing up and killing shitloads of people and then leaving does not win wars, it just LOOKS kinda like it does if you have no idea how to win wars, and assures you promotions in your organization. As soon as your military leadership starts citing that instead of actual progress on the conflict and the objectives at hand, it's a safe bet they are on their way to loss via attrition.

And that's not the ONLY factor of course, our military is too expensive and relies too much on fancy tech as opposed to solid strategy, everything we use is hideously expensive so any losses we take tend to hit harder, etc. But I think this is the most important thing to cite when discussing America's inability to actually wage war in a way that does anything besides get service people killed and enrich the MIC.