It is a bit unfair to compare the US NFL system to our AFL system because they have a truly national strategy to identify the best talent. They basically have three tiers of development that prepare young players for the senior teams. The first is their junior comp which you could loosely compare to our AFL Auskick and junior competitions. The major difference is that once the kids progress into the junior ranks from the Auskick "funball" competition, they are coached to their strengths by dedicated full time staff, not mum and dad volunteer coaches like the AFL.
As these kids go past the 15 year mark into the second tier of their training in high school, the training strength conditioning and strategic approach to the game goes to another level. College coaches are quite often picked from the strong high school teams and can go on to coach NFL teams. If you consider that most junior footballers in the AFL system are still being coached by Mum and dad volunteers you can see a gulf in talent development begin to emerge.
By the time these young players enter the college system, the third tier of development, at 19 they are physically mentally and strategically very knowledgeable about the game they are playing. They play another four years on average in this competition, being coached by NFL grade staff, strength and condition people and have access to the best medical and recovery systems you can imagine. By the time they enter the NFL draft they are physically mature, have been involved in a national travel roster to play games, in most cases have been to colleges that are outside of their home state( eliminating the "homesickness" factor) and are finely tuned athletes.
You know a hell of a lot more about these guys at 22/23 as individuals in their system come draft time than you do about a kid who is17/18 and done well in our draft combine. If you want to mitigate risk of picking up players in our draft, draft them later and develop a proper pathway into the seniors. AFL need to put more money into grass roots competitions and a proper structure, rather than just hope the best athletes in Australia are going to want to play AFL.