A large part of the value is not the software, but the knowledge built around the software, and that knowledge must be known and understood by someone (the developers). This includes everything from domain knowledge, the history of why things are the way they are, what failed and what succeeded, the technical idiosyncrasies, etc. The software is in a sense secondary, certainly secondary to business goals as its entire purpose is to serve business goals. Software is a means and an instrument, not an end.

But perverse incentives make churn attractive. Switching jobs is often the best way to raise your income, for example.