A more accurate phrasing might be "One reason staffers quit google's car project? The company structured their pay so that an intermediate project milestone was treated like an IPO event."
It's not that the employees got rich so they no longer cared about money, it's that they basically got their maximum bonus from Google, so they were better off taking their talents elsewhere to get the next big payout.
Given that their CFO discussed these payouts on the earnings call of a company with $6.6 billion in operating expenses, it sounds like these were entering the $100million range. Makes sense given that Google was trying to recruit and motivate people to work on startup projects, while overall Google market cap wasn't going to grow exponentially the way that a 2011 facebook or uber might. It probably wasn't a bad idea, they just executed in a way that only incentivized employees to get partway done.