It does, though. A single line might not change - but the world around it is always shifting. Libraries in frameworks, Operating Systems, drivers, external data structures - nothing stays static in the environment our code lives in. As long as code is running, it needs to be maintained. (Most of the time it can be negligible, sometimes serious...)
Most of the things you mentioned are a choice, not something inherent to the code as an artifact. If the external systems your code interfaces with change, you need to change your code. Other than that, updating your OS, libraries, drivers - these are all choices. You can not update them, and the code should run exactly as fine as it did when originally written. You can virtualize the environment to mitigate the risk of the underlying hardware platform going down or you. Code decay, to me, is mostly self-inflicted.
