The U.S. military likes big wars against big enemies, not messy little pissant wars, and this keeps the contractors happy and makes counterinsurgency a career killer for ambitious military professionals.Perhaps it's ignorant, wishful thinking on the neocons part, but also it very well could just be calculating politics since they know full well if they present the possibility of a long stay (due to insurgents) then they'd never gain the backing of the Amercian people -- and therefore Congress. It's all a knowing cover, a dance of sick manipulation.
But I think there's at least one other critical point to all this: nobody ever thinks these wars are going to last very long. The very act of fighting a counterinsurgency is an admission that you're going to be around for years, because that's how long insurgencies last. If your war planning — driven by neocon ideologues — is so wildly divorced from reality that you don't plan to be around for more than a few months, what's the point of even thinking about counterinsurgency?
The end result: Iraq (and Vietnam).