People are decapitated, because they're the direct object of "decapitation" – the verb describing the removal of the head from the body.

I disagree that "we mostly consider the head when talking about people". We might mostly consider the face when talking about identity, but what specifically we consider in a person is context-dependent.

Also, "decorporation" would indeed suggest the removal of the body – but from what? "Decapitation" makes sense because the head is a part of the body. "Decorporation" would require the body to be removed from a larger whole, if it's to follow the same latinate prefix pattern.