As for chemical weapons, people accidentally create chlorine gas all of the time. Aside from that, the reason you don't see more of this is because the really potent nerve agents and useful explosives (smokeless powders and beyond) have complex reactions that require trained chemists to produce.
It's a good thing most people aren't motivated to do that, but Aum Shinrikyo happened in our lifetimes.
I personally have the know how to create several things that are genuinely dangerous (like an EPFCG...I even have a copy of the 70s-era paper from LANL), but absolutely zero motivation to do so. It's not regulation that's stopping me.
You can build lasers that can blind people with the laser diode from your blu-ray player and people do it for fun. Nothing illegal about it here either and I've built a few.
[1]:https://www.discovermagazine.com/the-sciences/how-to-build-a... [2]:https://www.businessinsider.com/12-year-old-builds-nuclear-r... [3]:https://www.instructables.com/Build-A-Fusion-Reactor/ [4]:https://www.popularmechanics.com/science/a30917842/homemade-... [5]:https://www.foxnews.com/science/teen-builds-working-nuclear-... [6]:https://www.nationalgeographic.com/news/2015/07/150726-nucle...
Regulation really hasn't stopped any government from manufacturing nuclear weapons. Proliferation treaties have largely been a failure as you can see evidence of in recent years. The actual _use_ of nuclear weapons is something else and has nothing to do with regulation. The use of nuclear and chemical weapons doesn't support any governments war objectives. It turns out you don't actually win if everyone in the world simultaneously hates you, won't trade with you and will sanction you into oblivion. It turns out that the ability to import and export goods is worth more than the dead bodies of your enemies.