Yeah, and the two are linked. Seinfeld's focus on the process and craft meant he got exceptionally good at it. The money was a byproduct, not the goal. The motivation was intrinsic and about improving his craft. To which there is no end. So he continues.

This discussion with Seinfeld, Chris Rock, Louis CK and Ricky Gervais is worth watching for anyone interested in that aspect of comedy: https://www.youtube.com/watch?v=OKY6BGcx37k