The reason is that it works well enough, most of the time, for most people. If you are trying to lose weight, you do not need an exact formula to compute the derivative of your weight with respect to time; you need a mental model of your diet and metabolism that moves you toward your goal. CICO basically tells you to look at your two most important levers for doing that (how much you eat and how much you move) and adjust them until your weight moves in the right direction.

But people have an extremely poor mental model of calories out.

In particular there is a great misunderstanding with respect to the role of exercise. It’s not the calories you burn during the period of exercise, rather it’s the changes to your body that exercise causes that encourage weight loss.

You don't need to have a good model of calories out. Just keep track of calories in and your weight over time, and reduce calories in until you're losing weight at an acceptable rate.

All models are wrong, some models are usefull.

The (over)consumption industry is trying to hammer home a message that basically boils down to 'calories model is not perfect, please do not moderate your consumption'. If forced hey are ok with you consuming 'different', but not with consuming 'less'.

The results can be observed every day on every street.