Diets traditionally focus only on the short term. They have an end date. Healthy weight management isn’t meant to have an end date.
For example: Foods are classified as either all good or all bad. If the food is bad’ you must avoid it at all costs, and if the food is “good” you can eat lots of it. It’s either black or white with no gray in between.
Another example is Calorie counting: Calorie counting is boring, time-consuming, and often inaccurate. It also categorizes food strictly as numbers. I.e you’re forced to assign foods a numerical “grade” purely based on their caloric value.