We don't do rolling upgrades on steam boilers nor do we do it on our door locks. Nothing is perfect, we just need good enough and we are currently far away from good at all.

When it comes to software it hasn't always been the case. Before the Internet software was considered something stable and even now there are areas, like functional safety critical software, where this is true to some degree.

It's not that it can't be done.

It used to be expensive to a certain extent it will stay expensive, but modern languages, tooling and formal methods will hopefully mitigate that enough.