My guess, which is just a guess, is that the founders got tired of Google Search and advertising and Android and mobile hardware and privacy issues and regulatory issues so much that the “don’t be evil” motto was glaring at their faces and made them uncomfortable enough to step aside and let others run the show (or run things into the ground). This must have allowed them to enjoy their riches in relative peace. Somet…
This sounds like a just-so story. Just as likely they are running sensitive projects inside of Google that interface with the government. I am far more likely to believe that than a billionaire simply enjoying their money for its epicurean aspects. Attaining a billion dollars requires the will-to-power. That's not something that goes away.
How that relates to Page (& Brin) ... I don't know.