What is difference between “on a diet” and “in a diet”?

Question

What is difference between "on a diet" and "in a diet"?

I know that "on a diet" is correct expression.

But, I want to know the cause that "in a diet" is not correct.

And I wonder why "in a hurry" is correct and "on a hurry" is not correct.

Both "on" and "in" have as meaning of state, don't they?

Answer 1

No doubt, some dictionaries say that you can use these prepositions to refer to a state, but they also say that you use the preposition "on" to refer to something that you are using, eating, or drinking. So you use "on", not "in", for examples:

1. I am on a diet.
2. He is on antibiotics.
3. He is on drugs.
4. He is on the phone.