This is a big issue but if you look at the development teams like Penrith, they tend to prepare more young players for FG over a set period of time, making them more appealing to other teams. Take Sean O'Sullivan. Played a great cameo role with Penrith and his value went up and he got his FG spot.
It comes down to how teams develop their players and developing halves when you have an established half, isn't a huge focus as you're likely to stick with the Moses', Brooks, Keary's etc before giving these younger halves a chance.
That and teams have a fear of dropping their star half, even for several games, as the managers just shop them elsewhere and mediocrity never gets punished.