> This seems exactly what human civilisation has traditionally done: conserve physical energy until it's the right time to hunt, do some very intensive hunting, spend a lot of mental energy figuring out better tools and systems.

Civilization is pretty young when you compare it to the evolution of humans. I think you're also missing out the difference of energy spent when comparing hunting versus agriculture. Even when you're not hunting, there is still a lot of physical activity around it. Not to mention automation.