Runtime compilation is more of an optimization, we could store the function pointers in a closure or global array or something instead (and in fact probably should store them differently to be handle multiple drivers on windows correctly, but making that correct/efficient would probably be harder even though not many people would need it.)

In C/C++ you can call a function pointer exactly the same way as a normal function, so they usually just define a bunch of function pointers for the extension functions and initialize them all at once. Most people probably use a library like GLEW or GLEE for that.