Because there aren't any real-world implementations. Why bother with MMIX when you already know it'll never go beyond a toy "hello world" equivalent in an emula

Because there aren't any real-world implementations. Why bother with MMIX when you already know it'll never go beyond a toy "hello world" equivalent in an emulator? MMIX might be good for educational use in an academic setting, but literally anything else would be better if you want to have fun working with it. Nothing beats the magic of seeing your first program run on a real computer.