Why wouldn't it be like that?
I can think of lots of reasons why it wouldn't be like that. In baseball, a batter can't put the ball into play with their leg/arm, for example. A tennis player can't have a ball bounce off his/her body and back over the net in the midst of a match.
The main reasoning, I suppose, would be to eliminate ambiguity. Today, there is often confusion over what is a "distinct kicking motion", which hasn't reduced ambiguity much.
I haven't been keeping an eye out for it or anything, but I don't recall seeing any goals being called back because they went off a body as I've gone through the old game reports.
This is what I'm wondering about. In, say, the 1920s, did goals count if pucks went off attacking players' body parts?