Graphical user interfaces are full of symbols. Symbols need to be reduced to their essence. This helps avoid cluttering the user interface with meaningless distractions, and makes it easier for people to "read" the symbol and figure out the meaning of an interface element. Realistic details can get in the way of what you’re trying to communicate to your users.
The goal is not to make your user interface as realistic as possible. The goal is to add those details which help users identify what an element is, and how to interact with it, and to add no more than those details. UI elements are abstractions which convey concepts and ideas; they should retain only those details that are relevant to their purpose. UI elements are almost never representations of real things. Adding too much realism can cause confusion.