My personal feeling is that it just doesn't fit into the workflow. With C you always have a compilation step and thus a build process, so a code generation step fits quite naturally into your makefile (or other build script).

With Python there's normally no build step – you just run the program. Adding code generation means adding another step, which you might forget to run, leading to confusion. If you make the build step mandatory you lose some of the upside of dynamic languages.