So if understanding the equation about how humans can obtain and maintain a healthy diet/weight is as simple as you present here. Why doesn't everybody just do that?

You say yourself, it couldn't not work. And yet there's hundreds of thousands of people that say it didn't work. Explain them, are they lying?

I'd assert, the oversimplified explanation is misleading. It's only true in the same way that drinking cold water will help you lose more weight than warm water. True or not, reality seems to strongly suggest it's irrelevant