Why the compiler dosen't automatically include all the header files present in it's directory ?

Question

since no two functions have the same name in standard c++ libraries , there is no need of writing multiple header file names.

Answer 1

Certain header files are humongous, and when you say "all" header files, it's a great bigger deal than I can imagine.
Compilation time will be endless. Executable size may or may not be different, depending on the compiler and optimization flags used to compile. If unused header files contain #pragma or #define directives, they can altogether, severely affect run-time performance.