That's what is being done nowadays, counting calories.
The problem is that calorie consumption is not constant. It's more like household economy.
If you earn (eat) a lot every day, you will probably end up with a lot of savings (belly fat).
One way of getting rid of those savings (belly fat) is taking a lower paying job (dieting). The problem is that your savings don't magically dissappear, and you can make changes that allow you to keep your savings (fat), even with a lower income (daily calorie intake).
Another way you can get rid of your savings is just spending more daily (like exercise). The problem is that, if you have a good enough income (daily intake), and sizable savings, you will only lose capital (weight) in the long run, no sizable short term effect.
So, a fat person body works, in what respects to calories, like a financially savvy household. Going skinny would be like going broke. Some of us could benefit from a way to teach our bodies to do a bit worse in the calorie finance department. Could be a lot easier than just dieting, exercising or both.