Because the "engineering" part is literally all that other stuff which is actually quite hard. Just like a civil engineer doesn't pour concrete for a living, we don't write sort or string search algorithms for a living, we build solutions that solve company problems. Usually using someone else's well-built sort or search algorithm. When I need to write the latter, I reach for my Knuth books.

That said, I still bridle at the term "software engineer"; I mostly accord with Alan Kay's quote "Software engineering hasn't happened yet."