That kind of metaprogramming is just a manual substitute for deep constant propagation. Your optimiser needs to work across functions and compilations, however. It's also tricky for the optimiser to decide how deep to pursue this propagation (think of it as marking all functions as inline). This is why, for example, memcpy() is typically also a built-in function rather than just an exported library symbol. Knowing the context (alignment guarantees, fixed size) can enable some very significant optimisations.

Still, compilers are increasingly starting to do this.