That means they're preserving it and reinvesting it so more wealth can be created. This is a good thing.
People get rich by not consuming everything as soon as they get it. This is a great thing for everybody.
And it's not just rich that accumulate. A person making minimum wage and just saving the same amount withheld by the US social security tax would have hundreds of thousands of dollars accumulated if they kept it in the stock market for their entire working career.
I was curious about this a few years ago and put together this spreadsheet, using this tax rate, rate of return and minimum wage over a 40 year period.
https://docs.google.com/spreadsheets/d/1RlSfkW-DKXAXtXKvZJR5...