The only reason you can design a bridge beforehand is because (millions?) bridges have been built before so you can apply the lessons learned. Even if your bridge is "unusual", it will still be similar enough to older bridges so you don't have to invent the vast majority from scratch.
Other kinds of engineering don't have the luxory of leaning on the prior experience so much, simply because there is less of it. SpaceX's reusable rocket could not have been be fully designed before built, simply because nobody built a reusable rocket before. But it could be done through iteration, which is just another name for experimentation.
Software tends to be less like bridges and more like rockets... all of which falls within the spectrum of "engineering".