The Iraqi resistance is anything but disorganized. They have weapon-smuggling channels from other Arab states, they Al Queda and other religious organisations providing training and men. In fact, they have a pretty significant command-and-control structure, it's just that it's very branched and asymmetric.
But they didn't need a government to create that organization, did they?
They are pretty much a government. They have hierarchical leadership, they have coercive power, they impose taxes (not monetary perhaps, but definitely service). The fact that they are not de jure recognized doesn't really matter much. Hell, they have a strict fundamentalist ideology, which they enforce quite brutally. The Iraqi insurgency is definitely not a bunch of happy AnCap fellows.