Science says you're wrong: https://www.ncbi.nlm.nih.gov/pubmed/28765272

"risk for obesity is primarily determined by total calorie intake"

The macro nutrients certainly matter in the details, but not so much in the overall picture. It does mostly work like a gas tank, and the energy content in good is literally measured in [kJ].

Fat has a lot of stored energy, why on the planet, do people believe the body is able to create and store energy out of thin air?

And it exactly works the opposite way, the body will use the stored energy in the fat if there's not alternative.