Brand new JTACs need good, professional CAS (and FAC(A)) from experienced crews that know what they are doing. You don't want some dumbas brand new JTAC F-ing up the 9 line and forgetting to deconflict his 155mm suppression to a dumbass CAS solo RAG student who doesn't realize he's about to get into a very dangerous situation. Don't forget the negative training for both as they proceed to F-it up. It's a pain in the ass for the fleet to support EWTGLANT and PAC, but it beats having F'd up JTACS where it counts, and I know the schools appreciate having experienced aircrew overhead to help teach the JTACs/ FACs.
JTAC training also occurs at Fallon and takes advantage of having both visiting Air Wings and resident aircraft and aircrews experienced enough to work with students.