Requiring garbage collection is a language issue, not just an implementation issue. (Note that I do not think GC is a bad thing, but it matters a great deal in this context.) Making GC work in hard real-time system is a Hard Problem. I think you greatly underestimate how difficult it is, given your final sentence. Read the Metronome project I linked to.

You are correct that not many people write hard real-time code; it's generally found in such things as software controlling planes, power grids or power plants.