Do best friends exist?

So the older I get I hear people say they don't believe in best friends.

Why not? It just means you have several friends and one favorite one. If you really want to get kinky you can have more than one best friend.

So I wonder, because my bff claims that I am hers and her sister says there is no such thing as best friends. All I can think of is, wow who hurt you.

So, for you non believers of the bff association, please tell me why do you not believe in BFFs?