Yeah this is a real good point, a "diet" isn't something that your meant to do for a short period of time. A diet is the food you habitually eat, meaning you can be on said diet indefinitely. Most fad diets can't really fall under this definition because you can't sustainably eat them for years or even months at a time for various reasons, such as completely ignoring a given nutrient(s) or just being plain unhealthy (looking at you Jilly Juice). A good diet, like vegetarianism and keto, have actual health benefits while also being sustainable for possibly your whole life.