Because an hydraulic system can fail and we need a back-up. Once the Earth is surveyed, how would the coordinate system fail? The satellites might not be there or fail, but lat/longs are just there.
Perhaps, the RF should just roll into the ICAO and use WGS-84, there is no advantage to holding onto a datum that is just a duplicate of it.
BTW, I quite agree that the three systems are needed and should seamlessly integrate because GPS was never intended to be used as it is now being used. And that is one of the reasons for objections to it.
GF