You say "before" as if it's a thing of the past :) gcc is still not written as a library. clang became famous partly for providing a C compiler as a library.

Though gcc's case is specifically because of ideology. Even ignoring ideology, there used to be no need to write compilers as libraries, because the only binary that would use such a library was the compiler itself. The only purpose of the compiler was to be invoked from a shell / build script to compile code. That is changing with things like IDEs that need "compiler services" becoming more mainstream, as I said.