We must also remember that despite hailing from the region Graham was not a St. Helens lad. I've always thought non-local players are more likely to move because they do not have quite the same emotional connection. A St. Helens lad will have St. Helens relatives making it just that bit harder to move (not least because of the peer pressure they are likely to exert).
Eastmond is another (and possibly better example). He had already uprooted himself from his region just to play for Saints. And he certainly has no emotional connection to the club beyond professionalism (or lack thereof).
Of course, there are exceptions (namely Scully). But I think it underlines the crucial importance of junior development centred around local lads.