It's very usual for Bay Area people to pay 50%+ of their gross for housing, so new grads at FANG companies don't necessarily have a lot available after taxes, basic living expenses and debt repayment to save.
Also, FANG comp packages have significant equity components and IMO it would be foolish to rely on that portion of comp for anything important; you can treat vested RSUs as windfalls to add to the retirement savings but should not rely on RSU grants to meet your goal of x% savings rate. New grads will have nothing vested for a few years so their ability to save is especially limited during that time.