From what I observed, there are two factors at play that work to discourage this simple model:
1) The body is good at up and downregulating its calories use and cravings depending on what you eat.
2) The "calories in/out" approach doesn't depend on particular foods you eat, which means you can't sell your new magic fad diet without doing your best to discredit it.
1) Yes, which is why it gets much harder to lose weight the longer the diet lasts, which makes more people do it as they don't get any effect.
2) Everyone wants a magic bullet, but sip far there isn't one. On the other hand, specific diets and restrictions may be easier to keep and cause faster or more weight loss than others, as well as suppress appetite more or less.
> Is it regarded as useless in a dietetic context?
Yes. For example, it's why a dietitian will tell you to eat whole fruit, and tell you not to drink smoothies or juices (apart from very limited amounts).
1) The body is good at up and downregulating its calories use and cravings depending on what you eat.
2) The "calories in/out" approach doesn't depend on particular foods you eat, which means you can't sell your new magic fad diet without doing your best to discredit it.