Why do early childhood educators need knowledge on the human body?

Depends on what is meant by "knowledge". Everyone needs some extent of knowledge, from knowing how to stop bleeding to knowing one is sick.
You could argue that for the children they teach, the world is a very limited place; educational methods must revolve around the limits of this microcosm, and the body is a familiar reference point. Not unlike how an animal might be tamed - one must be keenly aware of its gestures, behaviours, thought process. Furthermore, might possibly have to spot unusual signs (ailment) on the child, if not diagnose them accurately, as they might be more likely to fail to communicate it meaningfully.
Other than that, in my experience at least, in the preschool age there weren't teachers for every subject, so one would need a general familiarity with all sorts of those.