PLEASE ANSWER THIS

That is how it is defined.

At times some standard practices are followed so that while communicating with each other it is possible to understand what others might have observed.

You friend tells you that the angle of incidence is 30 degrees. Now if you are talking about the angle between the incident ray and the normal whereas your friend is referring to the angle between the incident ray and the glass slab then the communication will be erroneous.

So the angle is defined in such a way that there is no ambiguity while referring to the angle.