Engineering projects do not change frequently and radically. They can't afford to - but more importantly - they don't need to, certainly not at anywhere the pace of software.
A bridge may undergo some changes and facelifts over a 200-year period, but it's still basically the same bridge. But there isn't much software that could last even a fraction that long and not be completely obsolete (without most of it's code getting replaced). In the case of a bridge, the underlying geography isn't going anywhere. That's just not the case with software - even if the platform still exists, the rapid pace of technology means that the software will stop being of any practical use.
A piece of software needs to keep evolving, or it's dead. The same just isn't true of most 200-year engineering projects.