In Vietnam, Iraq, wherever, we were/are fighting actual human beings. Beings that can think, plan, operate as units with some fairly advanced tactics and weapons.
They tried to infiltrate our FOBs in Iraq, use sniper attacks or mortars from afar. And they have only killed a relatively handful of us.
A shambling mass of brainless human flesh motivated purely by instinct? I'm sorry, that threat is insignificant compared to fighting actual human beings.