Why subscribe to calories in calories out when it is painfully, obviously wrong?

There are two ways to represent the statement "calories in calories out".

The first way assumes that if you eat X calories - then you will gain X calories-worth of fat. This is painfully and obviously wrong. One calorie (as measured in the lab) does not equate to one calorie of body fat. There are all sorts of complicated reasons why this is so.

The second way is to realise that if I ate no calories, and only expended them - I would have a net loss of fat. If I ate way too many calories,and expended none - I would have a net gain of fat.

So... 1 calorie in does not exactly equal 1 calorie of fat gained... but there is still obviously some correlation between intake of calories and the laying-on-of fat.

Therefore, if I restricted the intake of calories (ie reduced calories in) while maintaining the calories out... I would at some point expect to see a reduction in the amount of fat on my body. Likewise if I increased the calories going out (while maintaining the calories coming in) I would also see a reduction in my body fat.

Therefore "calories in... calories out" DOES work as long as you don't assume that "calories in - calories out == 0"

because THAT would be painfully and obviously wrong.