Why is not all libraries added altogether instead of adding them one by one

Question

why do we have to include all the libraries on by one. instead all of them can be added at once and then we can chose whatever function we want to. in case i got the term wrong then i am talking about the #include<cmath> etc stuff

Answer 1

Because with command #include<cmath> we don't include the library, but only a text header file with deklaratins names of functions and others symbols from the library,