Software is super complex and cheap to update. Engineering like this, however difficult, is not that complex and it's very expensive and difficult to update.
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.
When you say Engineering is not that complex, have you taken into account corrosive sea water, pressure, currents, what it means to make repairs and maintainance down there etc? It is difficult, because it deals with a very complex world full of physics, chemistry and even biology in a way that does not allow errors.
Engineering doesn't seem complex because there are centuries of learnings behind it. Those learnings become rules and suddenly it appears "simple" because no one debates whether to use wood or concrete when building an undersea tunnel!
....however that really is a kind of simplicity. Your training is relevant throughout your career. In software that is much less so. I think Comsci is a worthwhile degree but mine was really only a starting point.
Software does allow errors hence, IMO, we overload the complexity and "underload" the proof of correctness. We're not really that afraid of failures most of the time.
Software isn't inherently complex, it becomes complex. Because it is iterative. Because we keep making demands of it that weren't planned.
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.
Imagining building a bridge and then in the middle someone comes along and says it should also be a tunnel
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.
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.