Why white spaces are not considered as white spaces when we declare header file in c++?

When we declare header file in c++. It is not recommended to insert white spaces between angular braces '<' '>' and header file name. If we insert it comes out with an error. I have tried in Xcode and various other IDE's

// It is perfectly valid ( in c++ )
#include <iostream> 

but 

// It is invalid
#include < iostream >