Intersestingly, it works the other way round too. In my time as a military instructor, I have flown with two RAF students who had commercial licences prior to joining. One was great. A nice guy who enjoyed his time flying helicopters over the North Sea, but accepted that if he wanted to be a fighter pilot he was going to have to do all the early exercises again. As it is, he enjoyed it . He realised the style of flying was so different to his previous experience, he had to re-learn. He didn't find it taxing, he got on with it, did well and had a ball.
The other, who had a CPL form another country and who had done instruction and charter work was completely different. He had a real attitude about learning to fly a different way. He could not accept that he had to fly the JP and thought he should go to Valley on joining. He would not accept advice, and would frequently pick up his instructors on points he considered wrong - and their instructional techniques. I ended up flying with him as a lot of my collegues refused to fly with him. He only just scraped through the course as he was only a mediocre pilot - despite his (over) confidence.
JOAT. I think the answer lies in thet the world of military flying is different enough (even at the military transport level) to make the transition without significant retraining impractical. Although flying a Tristar from Brize Norton to the Falklands is no different from flying a B747 from Heathrow to Rio de Janeiro, as a Tristar pilot you would also have to conduct Air to Air Refuelling operations which without a background in military aviation, would take a while to learn - not just the operation, but the style of operation as well.
It's a case of old dogs and new tricks which in part makes the prospect unviable.