Fair enough. Upon investigation, I've discovered there are a fair few OS X official system shortcuts which utilize function keys.

But I think you're an edge case. Apple targets hard the average consumer, and there are plenty (my parents) who don't understand an arbitrary mapping of a number to a function. The self-explanatory icons (the speaker with lots of sound vs no sound, the universal play triangle, etc.) are far more understandable. So why not save space?