Matter of preference.  I think in general the throttle doesn't need a "fine" touch like prop and mixture.  There is also the issue of, for lack of a better word, fumbling.  On go around you don't want to push and forget to get the button.  I also know of one case where the throttle left at full during a start and was not retarded quickly causing a run off the runway.  Part of the cause was not getting the button pushed.

Dan Ballin
LEG2 #286