Software packages are more complicated than you make them out to be. Off the top of my head:

- license restrictions, relicensing

- patches, especially to fix CVEs, that break assumptions you made in your consumption of the package

- supply chain attacks

- sunsetting

There’s no real “set it and forget it” with software reuse. For that matter, there’s no “set it and forget it” in civil engineering either, it also requires monitoring and maintenance.

I have talked to colleagues who wrote software running on microcontrollers a decade ago, that software still runs fine. So yes there is set and forget software. And it is all around us, mostly in microcontrollers. But microcontrollers far outnumber classical computers (trivially: each classical computer or phone contain many microcontrollers such as SSD controllers, power management, wifi, ethernet, cellular,... And then you can add appliances, cars etc to that).

If something in software works and isn't internet connected it really is set and forget. And far too many things are being connected needlessly these days. I don't need or want an online washing machine or car.