1) Once you become rich you also have a fair bit of say in what it is that you do. If you are an hourly worker stocking shelves at Walmart your autonomy is limited and the chance that boredom sets in is much higher than it might be for most white collar employees. Where white collar employee's is a proxy for rich.
2) An hourly worker can only grow their income linearly with the number of hours that they work. The rich often have huge leverage with the time they put in vs the amount of money they get out. Sales is one example of this, a hedge fund employee might be another case. This tends to favour the "you eat what you kill" type of compensation.
3) The people I know who become rich often do so not by diversification but typically from concentrating on one endeavor, typically a company. In this case the company tends to become a very large part of their life. They aren't always at their desk but they are always thinking about their company. Maybe this is just an offshoot of point 2).