Good question, with a couple of possible answers:

1) maybe those species aren’t as unchanged as we think, because our evidence of the past is limited to things like skeletons, and maybe the skeletal structure of the shark didn’t change much, but other aspects did.

But more likely…

2) maybe unchanging requires a stable environment with stable selection pressures - but humans have had a rollercoaster of change in selection pressures, possibly more than any other species.