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.