[7thaccount]

What people fail to understand is that the rules/logic of a particular business that has been around for awhile can be absolutely enormous, complex, and poorly documented outside of the thousands of lines of code in production systems .A lot of this is due to historical choices that nobody remembers why a decision was made, but another problem is that many employees hate documenting what they know as it is either boring or they think it makes them less valuable, and management usually doesn't push it because it isn't a big quantifiable win on their resume and most aren't going to stick around long enough to see the fallout from the loss of institutional knowledge over time and turnover. Rewriting can be very challenging even with the domain knowledge, onboarding people to be competent in the business can take close to a decade... etc. They're kind of like programming languages where there is no spec other than the interpreter.