We take advantage of the situation. If we invented some way of e.g. "growing" structures that turned out to be much cheaper we'd probably adapt our attitude to changing them.
Vibe code a bridge! Arf. I am sure someone will.
Imagining building a bridge and then in the middle someone comes along and says it should also be a tunnel. I think therein lies a main difference to engineering and software engineering: planning and sticking to a plan.
Another thing are incentives: real engineering has real incentives to do it right, else you will get sued - by the families of those that died. Software engineering does not have this incentive to get it right.
While converting a bridge to tunnel mid-construction doesn't happen, what does often happen is that design assumes a particular construction technique can be used, construction starts with that technique, and midway through it's determined that an entirely different technique is required. This results in a bunch of redesign, remobilization, etc. Just like with software, construction often does not survive first contact with reality.