A lot of that has to do with the massive cost of maintenance (which can exceed the vehicles production cost EVERY YEAR). We don’t sell them to anyone anymore because that has bitten us in the butt a number of times (see also: Iran’s airforce), so once we are done using them in combat and/or training, they end up here.

As for the third world just catching up with our 50 year old tech–have you seen what proportion of our economy is spent on defense as opposed to the nearest competitor? There is a reason why no one else forks over enough to have what we take for granted.