tdr: For a seasoned industry insider such as you are, you should not really be shocked. Or, it is just a choice of words - unusual, difficult, I see your point there. I think the logic is crudely simple.
G/A is a dynamic manouevre, requiring just the usual amount of focused coordination. But in a slightly different workflow pattern. As a EU short haul pilot, I experienced 2 for every three years on the line and that seems to be pretty much the average of my peers too.
Your esteemed test pilot colleagues have GAs as a daily bread, and yet every mission profile I suppose had been polished first in the SIM. A weekly bread to me is G/S intercept from above to 7 miles in busy airfield when people, me included, sometimes try to overachieve. But not a G/A, that just does not happen often enough.
The devil in the detail lies inside the words "slightly different workflow pattern". Us average line Joes cannot rely on any seat of the pants or motoric action for croschecking nothing went amiss. Every item requires cognitive processing, even the FMA modes read different and your mouth would not spell them out automatically as it would for take-off.
The drill is just not there, and the typical amount of SIM G/As, that are mostly OEI requiring yet another slightly different pattern, is not enough to build one.