The defense for the civil engineer is that his design was in accordance with usual and customary engineering standards. If he did something unusual or new, he might be liable if that was the root cause of the failure. If he signed off on a sound design, he's probably OK.

Should work the same with software. The problem is that nobody learns that, schools don't teach it (school isn't even required to be a software developer), and there are no licencing bodies that set and enforce the standards. And, ultimately, most software failures don't cause death or injury.