This is the question reviewed here: Are plagues and wars the only ways to reduce inequality? | Aeon Essays. (It’s a long read but a good one.)
If you are not familiar with this idea, consider this graph:
The higher the red line is, the greater inequality is. Throughout the last 2000+ years, inequality has been reduced only by terrible events like plague and war.
For a time post World War II, inequality was declining in much of the world. Then, around the 1980s, it started to increase and continues to do so. Now we have a race on. Population declines should occur over the next 100 years, leading to greater equality. To counter that, we have greater automation occurring which may boost inequality as those with the means to control the automation make much of the income and increase their wealth. Will this inequality lead to events that once again levels off the distribution of wealth and income? Or will we reach a balance somehow?
I highly recommend the article. Rising inequality will be one of the great strains on the 21st century, and this article helps to provide some context on the subject.