Orks have not only survived, they have prospered and are more numerous than humanity. This at least is due in part to how they reproduce. Orks reproduce through the release of spores, which grow into a plant-like womb underground that nourishes the bodies of the various Orkoid species. This is the entire basis of the Orkoid ecosystem, producing first Squigs, then Snotlings who cultivate the Squigs and fungus, then Gretchin to build the settlements, and finally the Orks themselves. This means the Orks, where ever they go, will have an abundance of food, slaves and other resources, a moving ecosystem that supports them as they go on their Waaagh!s

This also makes it extremely difficult to rid a planet of Orks, even if the initial invasion is defeated. Orks release spores thought their lives, but release them particularly at the moment of death. Without a nearby population of Orks, the fungus will eventually start the Ork life cycle anew. Decades after weathering an Ork Waaagh! settlements on a planet can find themselves faced with an unexpected attack from Feral Ork tribes coming out of the wilderness.