Because the people who designed the library format didn't want to "waste" space for rarely used information like C preprocessor macros and function declarations.

Since you need that info to tell your compiler "this function is available later when the linker is doing its job", they had to come up with a second file where this shared information could be stored.

Most languages after C/C++ store this information in the output (Java bytecode, for example) or they don't use a precompiled format at all, get always distributed in source form and compile on the fly (Python, Perl).