Look to South Africa as an example of why things are happening the way they are. Simply going out and arresting your predecessor is a risky political move.
You can see how South Africa destabilized and mass fighting began in the streets very shortly after that happened.
This is why the plan is the way it is. Remember that.
Of course foreign enemies would sense this as weakness and strike. So the methodical and slow approach is what the US military wants to do while minimizing public panic.