When you are the boss work is a lot less unpleasant than when you are an employee. When you are at the highest level in the organization and also the major shareholder, you can shape your work environment and and workday in a way that you like it.

He worked for 60 years because he liked doing it, and as he became more successful, the job just getting more pleasant for him.

I cannot speak for him but from reading his annual reports and various writings and listening to the occasional interview, it seems that he enjoyed working much more than anything he would do while being retired.

You can call this great american work ethic, and that is part of it, but the other part of it is that when you are the boss you can kind of remove most unpleasant parts of your job and leave only the parts that are the most fun and interesting for you.