Physical systems work perfectly, until they don't. Roofs and bridges collapse sometimes, even though you'd think we would have figured them out by now. Rockets explode because materials behave slightly differently than the engineer expected.

Building things is really hard. I don't think software engineering is in any way special. It's just that the problems we solve with software are often more complicated than the problems we solve in meatspace and failure is much more benign so that QA is not done with as much diligence.