Well... I guess they don't as it yet as staking as a service is still a new thing... and to be brutally honest, STEEM just doesn't have the profit incentive for them at the moment as it is so low in price!

The top witnesses are a bit weird at the moment, as they are mostly unknown... but hopefully as the ecosystem develops then we will get community leaders or known developers who are actually contributing to the long term health of STEEM... there is nothing wrong with have exchanges also being there, but they do have a different interest in what it means for them.