Wikipedia has an article entitled "List of Diets". I counted about 75 different ones until I gave up, and there were still eight other categories left.
Here is my take on why there are so many:
1️. New Diets Sell
About 45 million Americans try a new diet every year, and 95% don't stick to it.
It sounds like a great market to capitalize on if you're thinking like a businessman.
That's why weight loss is now a 70 billion dollar industry in the U.S. Creating a new fad diet that promises to solve all of your problems with quick, easy solutions is a great way to make money.
2️. Principles Don't Sell
No one wants to hear that getting fit is a slow process that requires discipline, sacrifice, consistency, and many ups and downs. That's countercultural.
People would rather hear that they can get results by simply eliminating a food group or following a diet for only 30 days. That's more America's style.
3️. People Are Unique
Not all diets are "fad" diets created by someone looking to make money off our impatience.
Nutrition is an n=1, meaning that you should be the only subject of a study about the efficacy of a diet.
People have different dietary preferences based on nature and nurture. Our culture, emotional relationship to food, family upbringing, etc., influence our nutrition as much as our genetics and physiology.
Here is the main takeaway
Learn nutritional principles, build consistency with them, and add complexity later.
... but you probably won't need it.