The islander kids are a huge source or big athletes, both forwards and backs.
Another thing is that players don't generally start in the NRL until they are older. You just don't see many teenagers, and a lot don't debut until their early 20s - even players who become stars. One reason for this is that the development pathways include serious lower grade rugby - NSW Cup, Queensland Cup etc - where young players learn to play open age at a pretty decent standard for several years before most get a sniff of the NRL, as well as working on physical developemnt.
But fundamentally its a supply issue - RL is a huge sport in NSW and Queensland, and draws on the very best athletes, many of whom dream of playing NRL.
There are lots of bigger lads in the north of England who could make really good RL players, but aren't interested as they all want to be footballers or nothing at all. Odd how Wales seems to find big, fast lads to play rugby, but in RL heartlands its a real struggle - so many of our threequarters are just lightweight and not even that quick. How we tap into that resource is beyond me though....