Because computer science is not an engineering degree, and (at least in the states) professional engineers are required at a minimum to hold a four year degree in an accredited engineering program, work under a certified engineer for a period of time, take an exam, etc. While this is for literal physical engineering, there is not an appropriate parallel for software engineering. There are people who hold doctorates in computer science that cannot build functioning software. And it’s not expected that they can — CS is not an engineering degree. If mathematicians were commonly employed like software engineers, they may find themselves adopting a Mathematic Engineer, but that doesn’t mean it bears any relationship to an engineering degree, let alone any degree.