For me being rich is having enough money to last the rest of my life without working plus having no debt with my home paid off and self sustaining utilities. Then having enough money in an investment that gives me an income of a few thousand dollars a month to live on.
Once I hit that point I will retire.
why are people like Bill Gates, Elon Musk etc still working then

?
Well they still have enough money to stop working, some just choose to keep going. For some it's not just about the mopney but success or making a difference.