>solely on the amount of calories consumed vs burned.

It is, but CI\CO as advice doesn't take in to account that you aren't a machine and we aren't calculating gas mileage. The system is both adaptive and reactive. CI and CO changes based on the situation, and you have no way to accurately measure.

That's why you don't worry about Calories Out. You measure Calories In daily and then your weight at a sample rate that's long enough that it averages out most weirdness - probably weekly. If your weight is going up, reduce caloric consumption.