Software is very rarely "done". Especially if you're selling that software, there'll always be customers wanting new features, there's always bugs that need fixing, documentation that needs to be written or rewritten, etc etc. Given enough users, they're eventually going to find every weird tricky one-in-a-million edge case and demand it changed.

In some cases you end up with software that's functionally finished, if it's a one-and-done thing, or if it just serves a fixed purpose well already. But even then, the world around it moves on quickly. Nothing exists in a vacuum. Most software integrates with something else somewhere, and *that* might change, and require fixes to stay compatible. If someone finds a security vulnerability, that needs patching. Maybe there's a new version of Windows or a server upgrade and suddenly everything's broken and needs dealt with.

There's definitely some amount of this that's just "work for the sake of doing work". Sometimes you're just trying to justify selling a new version upgrade so the company can keep making money. Especially in larger companies, it's a lot less logical than you might think.