[peter422]

Modern industrial farming "wastes" tons of energy to get their desired outputs.

It's possible that the machines were advanced at some things but something that human bodies produced was something they couldn't directly synthesize.

[SilasX]

Right but that still means they need to pick literally any other human output besides “energy”. At the very least it would have to be some very specific form of energy that humans emit.

Just like it would make no sense to explain the purpose of animal agriculture in human society as “extracting the animals’ energy”. Like you said, it’s the different form of energy (plus nutrients) that they can provide, not energy simpliciter.