There are definitely other factors at play, but have no illusions about it: if the United States didn't mind killing and maiming a large number of innocent civilians, it could just knock down entire cities and wipe out entire populations if it wanted to. The main reason that those conflicts are tough is because the US tries to make a show that they care about human rights instead of just shelling the crap out of insurgents.

And the current struggles of the American military only result in hundreds or maybe thousands of American deaths, which historically, would be a mere rounding error. Afghanistan, Iraq, Syria, et. al. are a mess, but they don't come to a significant human cost to America.

The United States and its voting populace doesn't really care about whether or not developing countries are wartorn, and quality of life is decimated—America and other states have completely destroyed the backbone of several societies in the Middle East and caused the deaths and relocation of hundreds of thousands of innocents, but you don't feel the pain of that when you live states-side. So the country has little incentive to quickly resolve these conflicts besides the bad PR; ongoing conflicts mostly buy time for protracted, proxy diplomacy with other major powers to lay claim to natural resources, and aren't viewed as conflict with a tangible human cost.

In both those conflict it was mostly not the US being at war with Vietnam or Afghanistan but with them supporting one political faction within the country against another which is a much messier business.