Look at this from an economics perspective, do people in this sort of positions get paid more than comparable positions in other fields? I'd never heard of legal liability for software engineers. Has anyone ever been convicted or otherwise been held accountable when there was a failure?
The whole process from requirements, specifications, high and low level design, implementation, validation and verification and the rest of the lifecycle requires stringent oversight, including documentation and signoffs. The signatures on those documents have legal meaning and accountability for the engineers who did the analysis and review at each stage.