So the reason that there's so much space wasted is simply because IBM used a generic controller chip in the original IBM PC, and it never changed (even to this day), whereas more advanced machines like the Amiga had much lower level access, and could do clever things like change the length of the dead space between sectors, which were unchangeable on the 765.
The Amiga doesn't need any space between sectors because it can only read or write a whole track at a time. The controller chip in the PC is actually far more advanced than the rather brain dead controller in the Amiga.
