A lot of these net worth increases are from the rising value of companies whose shares they own a lot of (e.g. Amazon in Bezos’s case)
Many people aren’t interested in a life of leisure. They want to create, achieve, and strive. In the same way that someone may want to spend more time creating art, people like Bezos and Musk want to spend more time creating companies that build and make things. Both of them spend a lot of their time on passion projects having to do with space travel, for example.
So the question is: okay, you make your billion and you retire. What then? Lie around and watch TV and stuff? A lot of people are not interested in that.