You can't force someone to keep taking your money (that's indentured servitude), you can only incentivize them to stay with increasing amounts of money. Google almost certainly did do that. Probably by vesting his hiring bonus over 2-3 years.

OpenAI is in a unique position right now to grant pre-IPO options (probably in the form of RSUs). And they wanted him badly enough to grant the extra options necessary to effectively 'buy out' whatever unvested Google bonus he's walking away from.