The American dream link I added above was very marginal between countries. Even those where the bottom quartile economically had the highest opportunity to reach the top quartile, the chance was still barely over 20%.
So, from an economic perspective, even the best case scenario is:
-80% of those brought up in the top 75-100% will not reach the top 25%.
When you look at stats like that, if money motivates you, then you can see why so much economic migration occurs. If you have such a low chance of succeeding nationally then why not try and play the game of international wage (or benefits) arbitrage?
Edit to add: I bet the vast majority of those who do socially climb have to do it off their own backs (e.g. Entrepreneurially IN SPITE of the system) rather than via some kindly "Trading Places" benevolence.