He was rich enough not to need more money before starting YC-the acquisition price for viaweb was $49M, and it roughly ~10x'd between then and the peak in late 1999. (cites: https://www.cnet.com/news/yahoo-buys-viaweb-for-49-million/ https://venturebeat.com/2008/10/08/as-other-tech-stocks-rall... ). I don't know how long he spent there or what fraction of the company he still held, but I'd assume that he'd have stayed long enough to get to his personal Fuck You Money.

More generally, I know many many people (maybe literally tons by weight?) who continue to work long after they don't need to, usually because they're just deeply interested in the domain. I've seen this up close with my father, who continues to farm at 70, and with some founders who keep founding despite having more money than they will ever need. So when someone in that situation says they work on a thing because it is interesting to them, I generally believe them.