You are 100% allowed to make metal rockets.

The only "rules" preventing that is the Model Rocketry Association's Safety Codes, which is the same thing as the Comics Code Authority making sure there's no boobs in a comic book.

There are several hundred examples of people building rockets made out of metal on YouTube.

Even NFPA 1127 is just a guideline.

Only because there's an agreement between propellant vendors and NAR/Tripoli who want to police what you do (commercial organizations) - no statute exists prohibiting these activities (besides possibly ITAR and friends - which don't prohibit most domestic applications of this tech.)

Motors and batteries are available from a very large number of suppliers and used for many other applications - no such single chokepoint exists to control drone users/manufacturers.

There is no rule against a metal rocket per se. However, if you're going to go more than a few hundred feet up you enter the realm of the FAA--and you're a very badly behaved aircraft.

If you want to fly in their realm you either keep your craft light enough and soft enough that should you end up occupying the same bit of air as an airplane that you're not going to bring it down or you have to work with them. Since you can't be trusted not to run into things the only way to do it safely is to fire into an exclusion zone. The high power rocketry guys operate facilities out in the middle of nowhere and with scheduled launch days and a NOTAM will be issued keeping airplanes out of the space (middle of nowhere so the closed airspace is as minimally disruptive as possible.) The rocket guys are responsible for ensuring no rocket-on-rocket hits. Obviously, the FAA isn't going to be interested in handing out more of those exclusion zones than are actually needed, you want to send up amateur stuff, you work with the system.