Perhaps it's asking too much of the Founding Fathers to have anticipated modern weaponry when all their experience was with muskets. I'm quite sure I would not be up to proposing laws to govern use of technology 200 years in the future. Also, at the first census in 1790 the total US population was 3.9 million, of whom 20% were slaves, and mostly people lived in rural areas, not cities. School shootings and other mass murders were non-existent, and I doubt anyone could have anticipated the modern US fascination with guns for guns' sake. In the Founding Fathers day guns were utilitarian tools, like plows and axes, not objects of worship as they are to so many today. Our modern problem with guns is a creature of our own manufacture.
There is also a problem that it has become all but impossible to amend the constitution. Again that is not the Founding Fathers fault. They created a system to amend the constitution, and for centuries that was done not infrequently. It was never easy, but if an idea had widespread popular support it could be done. In the last 50+ years it has become effectively impossible to amend, largely due to entrenched political polarization in a two-party system. The interpretation of the 2nd amendment has changed radically in recent decades, away from a view that ties it to participation in militias towards an absolutist right of virtually anyone (including mentally ill people who legally cannot even manage their own financial affairs) to own weapons of almost any description. Most people support some limitations, such as background checks, but it is politically impossible to amend the 2nd to allow any constraints, even if they are widely supported.