It’s because it’s a diet not a lifestyle change.
Eat (insert fad food item) and nothing else = drop a load of weight. Return to “normal” and put it all back on.
Eat sensibly and exercise moderately and you will drop some weight and become fitter. To drop more weight, either eat less or move more.
There is a whole load of anomollies surrounding eat less / move more, that will give differing results for different people. Some are sustainable, some aren’t.
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