Because the toxins affect people other than the target. Some areas the US military has used for munitions testing have been declared Superfund sites. There are places in northern Europe so contaminated with arsenic and mercury from WWI munitions that they're predicted to be unsuitable for agricultural use for thousands of years.
What percentage of ammunition fired do you think kills people directly? And how many people do you think are exposed to that ammunition but not targeted by it?
It's sadly low in combat due to the chaos and lack of time to engage properly. It's similar to the police. It's something like 18-20% direct hits. What people who have never been military or police may not realize is that when your body goes into fight/flight mode, you lose your fine motor skills for gross motor skills. This is a very real thing. Training can compensate for this to a degree, but doesn't remove it. As the Corps used to say: "The more your sweat in peacetime the less you bleed in war." Also a person falls to the lowest level of training they have received during an incident. If you have developed great muscle memory and train hard, you should do fairly well. So many things come into play: learning how the weapon operates normally, how to tap/rack/bang, clear malfunctions, magazine changes, weapons maintenance--all these play a role in how well troops do when the chips are down.
