Exactly. Sometimes a player might be good enough to be first choice but has an outstanding player or two ahead of them. They're 'not making it' through misfortune not being through lack of ability.
Including misfortune of timing.
Ben Curry is intrinsicly nomworse a player than TCurry.
But Tom got 60-odd caps before Ben got started.
But that means that Tom has 60 caps worth of experience and comfort at international level, however many training camps to adjust, higher level (or just greater variety) of coaching and personal development.
Tom is currently the better player as a result of that. Tom is the starter, and Ben the depth player.
Because Ben got an injury when first on the fringes, and lost his place to Tom.
There's absolutely no reason to believe that it wouldn't have been exactly the same the other way around if not for that injury.
Maybe... I dunno, maybe Miles Reid (or any "insert name here") has an equally high ceiling as Underhill and TCurry, but coaches have never given them the chance, and the development, so he hasn't realised that potential, as is a depth player only. If ever given the chance, won't show the same, but due to that lack of development (and time to trust himself and the systems) to even do justice to his less-developed self.
Is "Insert Name Here" a depth player because the incumbent is exceptional? Or because the incumbent got there first, and avoided untimely injuries?
Were Ben Cohen & Mark Cueto really better wingers than Simpson-Daniel? Or did luck play a part?