Letting the clients handle the simulation would have a lot of problems for a MMO. Extra problems if you were doing the work for other players as well.

I think part of the point of it being a special emulated CPU set at a certain speed is to make it fair for each player, rather than the player having more processing power just because they have a better computer.

The easiest way to secure all that is to run it on the server. Anything going to the client or trusted coming from the client eventually gets hacked if there's much interest in doing so.