I saw a YouTube video where they said this was more-or-less the original backstory but then they changed it. I think it said that the People In Charge thought the 'living power source' would be easier for the audience to understand?

I don't have the link handy, and don't trust everything I read on the Internet, etc, etc.

But yeah - this makes so much more sense than breeding, raising, and feeding humans just to harvest their body heat.