Why do fewer people benefit from economic growth?

We often talk about this in the US. Looking at videos on youtube this seems to be common around the world. Most recently I saw a video about Paraguay and their recent economic growth and how it's benefitting few people.

An explanation that's often given about the US is that it's because all the growth is going to stocks and a small percentage of the population owns most of the stocks. I get the feeling that's an oversimplification of the situation. The stock price is a result of lots of people buying the stock or a few people buying lots of stock. Where does that money come from to buy the stock?

Now looking internationally, at Paraguay for instance, they talked about foreign direct investment, green energy investment(hydroelectric dam), and growth in tourism. You can more concretely see where the money is coming from.