Because nobody has those, nor had them, nor really wanted them. There were a few issues with popular models in circulation way back when, such as 1911s, but that was all relatively minor. Even back then, most were military-first models, and surplus.
Fully automatic weapons are owned and used primarily by rich people and gun shop owners, for fun, and by some police, for no good reason. Nobody ever owned rocket launchers or the like in any numbers, here. They have consistently been rare, outside of military hands. Just as the U.K. had very little gun ownership and crime even when guns were easy to come by, we've had very little in the way of ownership and crime with commercial explosive devices, even long before they were controlled.
While there are people that would like some repeal of those acts, nobody that wants to defend themselves, or kill others, is going to have much use for those kinds of weapons. Fully automatic weapons chew up ammo, and are going to be hard to aim (the purpose is generally not to kill, but to provide a threat of being hit by random fire, as it concerns single-man-portable guns). A single aimed bullet will do all the killing that a whole 30rd magazine would do. Grenade launchers and the grenades themselves add weight, bulk, and handling time that could get in someone's way, and serve little purpose outside of a war zone, except as it may concern flares, smoke bombs, flashbangs, etc.. The demand for those just isn't high, outside of police or military, though, because any of them draw lots of attention, even if they aren't made for killing, and without a group, could provide openings for any defending people. They've occasionally been used in crimes, but only rarely (even if you count police committing murder, and using them in the process).
Lever, pump, and semi-automatic loading firearms genuinely represent the pinnacle of offensive and defensive weaponry, for any individual, acting on their own*; whether it's for steel targets, deer, rabbits, squirrels, jugs, cans, or people. Therefore, they have maintained great popularity, are everywhere, and not enough people are willing to get into a hissy fit about certain features and tools they don't use having been practically banned to matter (suppressors being the one major exception).
If a crazy wants the best tools available to kill people with, compact semi-automatic pistols and carbines in common calibers are right behind bombs and fire, for the best tools to do it with. They're also among the best weapons to use for defense, for basically the same reasons.
The founders even realized that it was not feasible to guarantee that everyone would have, and know how to use, small arms; so that clause is left there to show purpose, and allow for it if the logistics could be solved. IE, that clause is the answer to why the rights shall not be infringed, and allowed the government, if means could be had, to enforce such requirements in the future (which to date, has not come, though we definitely have the means, nowadays).
* TV/movies like to make manual-loaders very slow for the sake of looking cool (big motions), and video games do it for balance purposes. They are slower than semi-auto, but not by that much.
** though I have serious reservations about the continued increase in armaments, and their use for situations where they aren't needed
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