Because they're not really useful. It's very difficult to keep chemical weapons contained so that they only damage your target and not your own soldiers or your allies or a bunch of civilians whose gruesome deaths will not play well in the media. In general, conventional weaponry is more effective at accomplishing military objectives.
Also because of a lack of imagination. Imagine using these puppies in large cities. The world lacks a better class of terrorists. Saudi Arabia used to do some wild stuff with planes back in 2001, prompting the United States to kill a million people in completely unrelated countries, but now we are back to car bombs and incels running with guns when we have a whole spectrum of biological, chemical and radioactive agen…
The reason we don't see more of this is pretty straightforward. Although we have active imaginations, the majority of people carrying out these attacks are not Walter White. It takes organizational power to pull off an attack like that.