I never understand why those who will never need any additional money still need to add to their wealth.

It ain't about the money per se. The money is merely a pat on their backs that they got it right (in the narrow, rarefied reasoning of their definition of success).

In many respects, Musk has a much higher definition of success. to rid the planet of ICE cars, and trucks. And he may well be dead before that is near total. Yea, I know he is going to space, but even there he is far more successful.