Ground idle takes about 5% of the total rpm, flight idle takes about 85% of the total rpm. Why is so much of the engine power wasted before it gets to flight idle?
Because flight idle is used in flight, and hence needs to provide enough thrust (or power) to be useful, such as sustain level flight at some parts of the flight envelope, under certain conditions (e.g. low vehicle weights). On the other hand, ground idle only needs enough power to enable taxing.