It's actually a question of coaching efficacy.
In New Zealand Cricket, very few youth are being coached in the right way by someone who encourages the right technique, plans and patterns of play. Arguably, the first time these guys receive any captaincy is when they've made their age group teams and by then it's either too late to change massive flaws in technique, or the coaches they have at provincial level aren't good enough. Most of the time rep players are those that naturally stood out rather than worked hard, and in some instances are only rep players because they went to the right school
In New Zealand Rugby, all youth have a coach that understands the game, most high schools regardless of where they're at have specialist back/forward coaches who can teach the technique as well as mental skills required to dominate in the game and by the time they make representative squads, they're only in need of a touch of polish to take themselves further, they've also climbed to the top tier both through a mixture of athletics and work ethic. A lot of rural schools contribute to the All Blacks because of this.
In South Africa, academies and such for cricket are much more prevalent, coaches are at a much higher standard and the expectation put on younger players around how they play their cricket is much higher. In Rugby, they still have good coaching, but not to the level of NZ.
