[aragilar]

One could flip that question around: given there's all these fast C/C++/Fortran libraries, why do we have all these scripting languages?

The answer usually is there's a bunch of IO/parsing/configuration that we need to do, and doing that in a lower-level language isn't that much faster overall, is much less maintainable or accessible for users than wrapping in a scripting language, or simply avoids the need for recompiling each time you want to modify something (e.g. game engines with a high-level scripting component).