But that doesn't make it necessarily the most useful model for a complex animal to modify its own behaviour. We are dynamic systems that can voluntarily and involuntarily vary the amount of energy we expend. Our bodies send appetite signals to our consciousness, which can be pretty hard to ignore.
The only times I've consistently lost weight has been via a calorie deficit. But it's totally understandable that people want another model, either something like keto which purportedly produces weight loss without a calorie deficit by messing with the body's fat-storage mechanics in some way, or some behaviour+diet which produces a calorie deficit but with minimum lethargy and hunger. There's a lot of nonsense around dieting but the basic desire is understandable.