Physical engineers face unknowns all the time. Building in unknown environments for example. New materials. Climate change. I can't even count the number of different designs for bridges, buildings, and cranes.
And once you build your bridge, you're finished. It just has to stand there and take the load. It doesn't have to be unpredictably changed over the course of it's lifetime to also function as an airstrip and shopping mall.
Even the simplest of real-world software products you typically need to grow and adapt - sometimes dramatically - over time.