There are many double blind studies on this. On a 2+ year time horizon, the huge majority of obese people who follow a "eat less exercise more" regime lose weight and then gain it back. This happens because the people never stop being hungry due to the disruption that obesity puts on their endocrine system. There are a number of medicines (saxenda, liraglutide, ozempic, mounjaro, wegovy) that produce reliable robust weight loss on a 2+ year time horizon. These medicines work best with a change in diet and exercise habits, but reliable weight loss on a population wide scale is not something you treat with "diet and exercise" advice.

Sure a caloric deficit will produce weight loss. Obviously. But reducing calorie intake alone does not guarantee a calorie deficit. While there is of course some level of unavoidable caloric expenditure, the average person has a fair bit of expenditure that is quite avoidable.

It is not hard for the body to discourage unnecessary caloric expenditure in response to a notable caloric deficit. Indeed, doing just that is a basic famine survival instinct.

Now given the decidedly non-famine conditions, eventually things should restablize, but it does mean for a person at caloric equilibrium, reducing caloric intake by say 500 Calories, will often not result in a 500 calorie deficit, but a smaller one for for quite some time until the body adjusts to this being the new normal.

In the mean time, the person likely feels like shit. Furthermore, If there actually is food available, the body and subconscious mind is doing everything it can to encourage the person to eat more.

This substantial resistance of the body to attempts to run meaningful caloric deficits for an extended period are a not insignificant portion of why "dieting" often fails. People don't stick with it, since it majorly sucks.