The art of healing is not particularly widespread among the Death Eaters. We have healers as well, but most of them don't serve us voluntarily. At St. Mungo's we are still treated grudgingly, even though the hospital has officially been under the Dark Lord's control for years. And he, of all bastards, has never shown any sympathy for his injured fighters, insofar as they managed to return to him alive at all. Being injured means weakness. A condition the Dark Lord does not even remotely tolerate.