A lot of times they move on with larger technical debt, less understanding of their own systems, and less resiliency when things start to break. These issues accumulate over time especially if smooth transitions between technical knowledge isn't kept (like when employees are constantly jumping ship), and lead to places eventually hiring consultants for $1600 an hour to fix their technical mess.

It also depends how regimented the team is, places like Amazon have large numbers of coders on the same project, watching over each other. But they also compensate their employees well.