Basically, because most civil pilots have never been subjected to the rigorous aptitude testing essential for military flying. How would anyone know whether a plain vanilla civvy had the additional levels of capacity required for effective formation management and the frequent in-flight mission re-planning demands essential in the AAR environment?
Many ex-truckies found AAR quite difficult to start with - and they were all experienced military aircrew who had demonstrated the required aptitude before being selected. And at least they knew how to l00kout of the flight deck windows when turning in close formation!
Of course there are many very capable civilian pilots. But someone whose has never been 'selected' and who has just self-funded their right hand seat postion with a LoCo could well be a considerable risk.
In any case, with shrinking RAF force levels, there would probably be plenty of well-qualified QSPs with AAR experience available to fill the 'reservist' slots with ATr.