It is simple as in you know if its a problem for you or not. Most people can eat sugar without becoming fat.

Yup. If more calories were a "simple problem to solve", how come it isn't solved?

It is a simple problem to solve. Eat less, exercise more. Tons of people have done it successfully. Tons of people do it without even thinking about it.

It’s just that some people have mental addictions or whatever problems that makes them want to overeat or not exercise enough. But the solution itself is simple and straightforward.

If the "tons of people" in the first paragraph were multiples more than the "some people" in the second paragraph, then I would agree that the solution is simple. But the thing you describe in the second paragraph is what makes actually solving obesity hard. Using artificial sweeteners is one of the ways in countering the latter half of the puzzle.

Fair enough. To me, simple and easy are not the same thing. Chess is a simple game that's not easy to play. In the same vein, to me, "take in less calories than you burn" is a simple concept, but not easy to do. It doesn't make the concept wrong, though.

"primitive solution"