I feel like proprietary software is made by someone that left the company shortly after the project was finished. They never documented in the knowledge management system, so no other employee is able to continue with it.
left the company shortly after the project was finished.
That's a short way of saying: the original team was partially downsized, primary functions were offshored to a disengaged team in a different timezone, and a bunch of conflicting requirements were piled on to version 2.
It's amazing how much work goes into cleaning up code before you feel comfortable posting it to a mailing list that you would never even bother doing for internal-only stuff.