A friend explained this to me with an analogy.

"If you make more money than you spend, you will have a lot of money!"

That statement is tautological, and obvious. But it doesn't address how to become rich.

Now let's compare.

"If you eat more calories than you expend, you will gain weight"

Again, this is likely true, but just very very incomplete. It doesn't attempt to truly get at the why. There are a few hypotheses for why CICO often does such a poor job at explaining the obesity epidemic. The two pieces I find most convincing are (1) there is some metabolic advantage based on the types of foods you eat, and (2) some of the calories out are excreted via waste and not processed by the body, and therefore, the simple CICO equation isn't complex enough.