Those are the grunts of the Army and make up like 90% of the Army. Easy to kill but also Easy to replenish. Vampires are higher ranking because of how much harder they are to 'make'. You'd need a Pure-blooded Vampire to infect another who isn't already Undead and turn them yada yada... it's a whole process to replace a Vampire lolZombies and skeletons too?? Man they don't stop coming.