Speaking from personal experience, execs (most of them) are anything but risk takers. They are good wheeler-dealers, know how to navigate the land-mine filled org-chart to get things done. Not too different from civil servants. Those who develop risk appetite won't stay at such big orgs, they either go work at a startup or start on their own.

So over a period (FB has been around for like 15 years now) an org accumulates extremely risk averse execs.