Because the parser still has to skip it, especially if it's in a file with code that is used. The parser needs to know the beginning and the end to do so. This

Because the parser still has to skip it, especially if it's in a file with code that is used. The parser needs to know the beginning and the end to do so. This takes at least rudimentary parsing. Oh, and the compiler may not know it's unused until it processes all files if it's a public function.