It comes from the halting problem. People can be more careful writing code, but it is impossible to be certain about all the things code will or won't do. Even very simple programs can have flaws that get found and fixed years later. It happens all the time.
We need to be open and honest about the possibility that our code may act in ways we don't foresee.
> If this was actually true, we wouldn't have safety critical software systems that have been running for decades without fatal bugs.
Not hitting a bug is not the same as not having a bug. I'd bet money that whatever system you're talking about has bugs. Plus, the system may be far simpler than you assume.
We need to be open and honest about the possibility that our code may act in ways we don't foresee.