Because the violence they participate in is fundamentally pointless, and then they come home to a society that is irreconcilably divorced from and indifferent to it.

i.e. - nobody around them understands what the hell they did and went through over there, and they have no idea why the hell they even did it themselves.

It isn't my job to bankroll Iran changing, and you're going to ship the guy who did that acid attack to Ohio when your war displaces him and call me a racist for wanting him out after he does it again. Fuck you.