There’s issues with the simple calorie in - calorie out theory (there’s research showing that different bodies digest food differently so multiple people eating the exact same food might not absorb the same amount of calories, and also, all calories are not the same. You can eat 2000 calories in fries and 2000 calories in balanced meals and your body will respond very differently).

But for the sake of argument let’s assume, as you claim, that calories in - calories out is all that matters. Especially if you make that assumption then “Just eat less” is a nonsensical piece of advice. Because “Just eat less” simply restates the problem, which is people eating too much.

Actually useful advice would require helping a person with how they can eat less, and helping resolve the issues that lead them to eating more than they should eat.