Even if it wasn't all within the software (which I would strongly doubt), the problem with safety and of complex systems is that the safer you try to make it, the more complex the system becomes and that in itself reduces the reliability and safety of the system.
If you add a monitor to check to see if a critical piece is running, then you need another to check if that is running, etc... This applies to both hardware and software. Then you layer on top of all the features of the system layers of process like CMMI, ISO whatever, etc. and designs are reviewed by committee after committee and eventually the design has to be approved and the cost keeps skyrocketing.
