Why can software not be finished?

I once wrote a Rs-232 translator softeare converting between two different flavors of a control signal. This thing is running for 15 years without update and a single bug now.

Granted it also isn't user facing and it's environment won't change, so there is no pressure for changes, but why do we think software needs constant updates? Because y'all use dependencies you need to update? Because you're used to software shipping half finished?