Because the software ecosystem is not static.
People want your software to have more features, be more secure, and be more performant. So you and every one of your competitors are on an update treadmill. If you ARE standing (aka being stable) on the treadmill, you'll fall off.
If you are on the treadmill you are accumulating code, features, and bug fixes, until you either get too big to maintain or a faster competitor emerges, and people flock to it.
Solving this is just as easy as proving all your code is exactly as people wanted AND making sure people don't want anything more ever.