I’m with you on that. Only things I can figure are 1) it might have not looked good to fire Comey now and 2) Comey knows a lot and may come in handy. In addition, I suspect Trump will keep him on a short leash and demand info and results and may put his own pick in second place, etc. in that agency.
I suspect those are two key reasons President Trump asked Comey to stay.
Now watch the left spin this as 'payback' for Comey reopening the Hillary investigation a week before election day.