Earlier quoted context omitted.
As self reported by many truly rich folks, money is not a motivating factor in their daily lives. If they want a good burger, they'll get one and not care if it's $1 or $1,000. Such an attitude would certainly apply to their reasons for working.
If you factor in the opportunity cost of a rich persons time, the cost of the burger itself matter proportionally less.
After all, there's a practical limit on the value of a person's labor - let's postulate that it's around $7,000 a day - there's no such limit on what investments can return in a day.
* $7,000 is Elon Musks's $2.2M salary from 2019 divided into 340 working days.