Isn't SV culture encourages to not stay more then 2 years in one place and always keep an eye for an opportunity, how do you screen for people that are a "risk"

Isn't SV culture encourages to not stay more then 2 years in one place and always keep an eye for an opportunity, how do you screen for people that are a "risk" of leaving not at home to take care of a child but at a new company to get a salary bump and work on something new and shiny?