Yes, the theory behind weight loss and maintenance is very simple. As with many theories, however, it's the application that's the problem.
I've lost and maintained 150lb weight loss until recently, when circumstances and stress has caused me to gain back some portion of that I haven't been keen to measure. I can tell you from significant first hand experience that it is very difficult to override your brain when your brain thinks it needs more food.
Actually, it is harder to do in the US than in Europe due to the insane amount of sugar they put into everything (even bread!).
Also, keeping oneself to a diet is a habit change, which is like the hardest thing to do, so don’t think lowly of those who have some trouble keeping themselves to it.
In the end, many revolutionary developments come down to us realizing the simplicity of things. Quantum mechanics is complicated. Eating well is simple. One just has to "get it".
It's like with shoes. If you buy cheap shoes, you end up paying more for shoes overall because they break much faster and you have to buy yet another pair. If you eat bad because it's cheaper/more convenient/whatever, you end up paying more for that decision than if you had made the right decision in the first place.
Repeating the mantra of "it's not that simple" is literally a self-actualizing curse. By thinking so, you make it so in your mind, which causes you not to act. My philosophy is that things really are simple.
Yesterday I ate nothing but ice cream. I wanted it, I bought it, I ate it. And it was stupid. There's some back and forth between competing desires, but eventually you will settle on the correct course of action. Every single substance and habit, no matter how addictive, I've started and also stopped. The key is literally just willpower. And maybe a bit of math. You have to count the goods and the bads to arrive at a conclusion. Ice cream tastes less good than it makes me feel ill for overconsumption therefore DON'T BUY ICE CREAM. Problem solved. If you then desire ice cream again, remember not how good it tastes but how ill it made you afterwards. What is it anyway? Cream and sugar. Sugar is a drug, so focusing on the cream it's probably an indication that my diet is lacking in fat. Eat more fat. What contains fat? Nuts, for example. Next time you desire ice cream, eat nuts. Problem solved.
How do people struggle with such a basic aspect of human existence? Does food packaging in the US not list ingredients or something?