That's how long it takes before the person who signed off on it has moved to another job and made the product someone else's problem. At that point the new person will usually kick off a new project because the old one is bad and releasing a new product is better for their chances of promotion

I worked at a software shop with very high retention rates (like, 30 years, 10 years on average), and the inverse can also be an issue, "I own this product and it's my problem not yours." Having seen both situations, I personally believe that it can also be that a new project comes to exist simply because the old one is too complicated to understand; some things you need to work through in order to get. Someone here on HN said recently, "people forget that the primary job of the software engineer is as a learning agent for the org" or similar, and the more I see, the more I believe it. I used to think it was all about efficient automation, but I'm not so sure anymore.