If you've actually played cricket (or any other team sport) then you would know that the laws of any sport are only a starting point for learning about it, and there's a hell of a lot more to how a team works together on (and off) the field than that. Different teams within the same sport work differently, and that's emphasised in interdependent team sports like cricket as opposed to independent team sports where there's little complexity beyond each member performing their personal role. A team that is short on star power but works well together can get good results that you wouldn't expect based on a pure head-to-head comparison of the players - look at Pakistan's victory in the Test series in England last year for a great cricket example, or the Western Bulldogs premiership team for an example from another sport.
Going back to the wicketkeeper example, here's a handful of the relevant points that an incoming player would need to pick up:
How does Smith communicate bowling tactics to the rest of the team, compared to the player's normal state captain?
How involved does the wicketkeeper get in setting the field?
Who shines the ball?
What signals are used between the field and the dressing room?
How does the group field as a team?
What are the team norms in the times spent off the field during a Test?
That's all stuff that Paine had a couple of weeks to get used to during training and team meetings in the leadup to the first Test, but which a last-minute replacement would have to try and pick up very quickly. Renshaw and Handscomb were intentionally selected when they came into the side last year and had nearly a week to get used to the team environment, they didn't come in at no notice as a backup wicketkeeper would have to in the event of a late injury.