Thomas Jefferson[1] wanted the Constitution to be rewritten every 19 years as he did not believe future generations should be constrained by the limited understanding of the past. Given that past history, the government and it’s various bodies didn’t have a problem with banning weapons more powerful than the founders could have imagined from being in the hands of every civilian.
What is probably confusing you is there is a long simmering but recently rapidly growing religion in the US[2] that does believe that the Constitution is sacred and cannot be altered, and have gained enough political and social power to treat it as such. If I recall correctly the United States is currently in the longest stretch between Constitutional amendments in its history, as a result of this political/religious movement
[1] one of the more famous founding fathers and a major contributor to the writing of the Constitution
[2] https://en.m.wikipedia.org/wiki/American_civil_religion for the simmering portion and Q as the rapidly growing portion