If Google says they'll give you $200k per year in cash + $400k split over 4 years as RSUs, that's the same as just getting $300k per year in cash.
Why would you leave $300k per year in cash on the table? Because another company is offering you $350k per year (or more likely in the case of FAANG, another company is offering you the same $300k/yr but with significantly less stress).