Because they're not yanking yet and the equity growth is still attractive enough to keep unvested "low performers" from leaving. Google don't have to cut costs to make Wall Street happy. They need to grow and increase the top-line, and for that, they need people.
Once they reach the point of "maturity" where they need to minimize operating costs to increase net margins and continue to justify their P/E, they'll be like Cisco, Microsoft, Intel, and others, disgorging 5-30% of their workforce in a single massive push.
Once they reach the point of "maturity" where they need to minimize operating costs to increase net margins and continue to justify their P/E, they'll be like Cisco, Microsoft, Intel, and others, disgorging 5-30% of their workforce in a single massive push.