Because it's the space-time that's expanding and not the matter within. It's the same principle which would allow an Alcubierre drive to achieve FTL.
It's also the difference between relativistic redshift and the cosmological redshift (even though both produce the same effect, the underlying cause is fundamentalistically different)
When the stars threw down their spears
And watered heaven with their tears:
Did he smile his work to see?
Did he who made the lamb make thee?