I think it's mainly because software engineering is still more an art than a science.
The use of the word "still" here implies that it will be, or that it should be, more "science" than "art". As an act of creation and navigating ever changing constraints, I think it is more art, and will have art be a major component of it for a long time, if not forever.
Although we may have formal engineering education, most of us end up making business logic where there are few rules on how to do it. You can't compare it with building a bridge, which has a strict mathematical workflow.