Why don't men of science have a code of ethics...?

We do operate under a code of ethics. It is inherent to the philosophy of science; it is a tenet of the philosophy, the idea of scientific investigation, that the scientist has to be honest (or he is not a real scientist by definition).  It is also coded in law for many professional practitioners of science.  I am subject to such a legal code as a registered and licensed geologist, in exactly the same way that a "doctor" (physician) is.

Scientific misconduct (as in violation of the ethical principles of science rather than in law) is not unheard of.  Usually, the punishment is removal from staff or removal of publications from journals, or removal of funding and getting blacklisted from future employment, funding, membership in scientific organizations, and publication.

Science has a code of ethics.  It is not a famous one like the Hippocratic oath, but it is a "gentlemen's agreement" of sorts due to the historical way that science came to be practiced.  Reputation is important.  Nevertheless, different professional organizations do often have some sort of written code of ethics. You are subject to such a code if you are a member, and most scientists are members of one or several organizations and at minimum, subject to those codes.

Your lack of knowledge of such codes does not mean they do not exist.

Medical doctors (which do not necessarily have a PhD) do indeed take an oath. 

Scientists (who almost always have a academic doctorate degree) in effect have the same: when receiving their doctorate degree they are knowingly and willingly bound to the ethics of scientific research.

Your specific example points more to technical engineering. That uses scientific discoveries, but isn’t science (applied science at best).Engineers are not scientists, so you are barking up the wrong tree I’m afraid...

Hominids were whacking each other with clubs for 100,000’s of years. If we didn’t have technology that had the potential to kill, we wouldn’t have technology at all. We would just go find more clubs.