>so why can those chemicals only come from things that were living?
Because food is a way to consume order. In Schrödinger's terms, the food chain is "life feeding on negative entropy". As you go up the food chain more and more complex organisms need to consume more low entropy things to maintain their more and more sophisticated internal structure. There's more energy in the matter of a rock than you'll ever need, but you can't gnaw on the thing to sustain yourself.
That said "life" doesn't necessarily mean "sentient animal". You can certainly expend energy to create artificial food sources but they'll always be lifelike, that is ordered for the reason Schrödinger lays out.
Because food is a way to consume order. In Schrödinger's terms, the food chain is "life feeding on negative entropy". As you go up the food chain more and more complex organisms need to consume more low entropy things to maintain their more and more sophisticated internal structure. There's more energy in the matter of a rock than you'll ever need, but you can't gnaw on the thing to sustain yourself.
That said "life" doesn't necessarily mean "sentient animal". You can certainly expend energy to create artificial food sources but they'll always be lifelike, that is ordered for the reason Schrödinger lays out.