Why do people say "the poor get poorer, the rich get richer"? Even though there are examples of poor or disadvantaged people who became fairly well-to-do, or sometimes even rich?

This video puts the advantages of privilege much better than words can:

https://www.youtube.com/watch?v=4K5fbQ1-zps

There are always exceptions to the norm.

Rich can become poor. Poor can become rich. Not majority but there will be a few.

Same like entrepreneurs vs employees. Not all can be billionaire entrepreneurs but some do make it.​​​
There is even a chinese saying Wealth never survives three generations