Why Chemical Attacks Are Rare

Explaining why chemical weapon attacks by terrorists are rare due to the complications of manufacturing and dispersing chemical weapons properly, compared to the ease of using a vehicle or knife to cause mass slaughter.
Chemical weapons were effective in the First World War as they were produced on mass by heavy industry and dispersed on mass against dug-in infantry with nowhere to run.
This is almost the exact opposite of the conditions required to attempt to make chemical weapons at home and then distribute them effectively.