Sorry, i do not understand. Could you elaborate? Is it a money issue?
If they are very good, like the people i think you and i have in mind, why can't they make money playing for a club, but still play for their country (and make no extra money out of that)?
To be crystal clear, i wouldn't mind, at all, if a player picked club over country. His choice and i am fine with that. What i do have a problem with is players getting to cherry pick which country they can play for.
First off, there's the extra money.
Then as the Mole notes, being eligible for whichever country it is tends to be good for your club, which tends to be good for your career.
Finally - a guy picks England (or Aus, or NZ) over Fiji, he plays at the highest level of rugby. He has a shot at winning the World Cup. He'll play other Tier 1 countries a lot more. Any guy that goes into rugby thinking "I want to be the best", he wants to play for a Tier 1 country.
There's a lot of incentives for a player from a Tier 2 nation to play for a Tier nation.