What makes you believe that they are "artificially" "frozen"?
It is unknown whether most D&D worlds have reached the level of metallurgy that allows casting or boring firearms to be manufactured cheaply. Why do you feel that D&D worlds being at this level of technology is "artificial" compared to a different level of advancement?
Usually with a loud "Clang!" . . .
The PHB
does address how D&D firearms impact armour: They have a chance of bypassing armour completely based on the attack roll. Given that even heavy cloth was able to stop the more advanced Napoleonic muskets, and shields were still used effectively against firearms, why do you think that they would do so differently from any other attack?
I mean the belief that bullets are "near instant-fatality machines" is just as hollywood-driven as people being able to shrug off wounds for a while, if not more so.