Professional engineers are held liable for mistakes in their work. If a house falls or a dam breaks the responsibility falls on them to show it wasn't built as

Professional engineers are held liable for mistakes in their work. If a house falls or a dam breaks the responsibility falls on them to show it wasn't built as designed or there was some other factor involved that a professional engineer would not be expected to have considered in their design. And, yes, as others have mentioned they get insurance, but there's more to it than that. The level of verification that those designs go through before being released is on a completely different level than what is usually applied to software.