metal fires often burn at more than 5000 degrees F. That’s hot enough to disassemble water into its component parts, and one of those parts is hydrogen gas, which is not only flammable but explosive. any uncontrolled release of liquid into the fire would be catastrophic. Metal fires cannot generally be quickly extinguished in an emergency or uncontrolled accident.
metal fires also release toxic gasses and byproducts that often require more consideration than electric or gas.
as an update to a few questions: NEVER add water to a metal fire. it will cause an explosion.
depriving the fire of oxygen works, but only insofar as it remains deprived until the fuel source cools from 5000 degrees, or it risks spontaneous reignition. it generally has to be monitored similar to a crucible as it cools.
most accidental metal fires do not have a cogent or quick option to deprive the fuel source of air.