I have had the unfortunate experience of seeing too many old dogs euthanized with injections. It isn't always quick, easy and painless. It's hard enough for me to watch an old suffering dying dog get euthanized when I know it is necessary. It would be unbearable for me to watch as healthy young dogs were injected and I could not do that job for any amount of pay.
I imagine that if it was your job to euthanize thousands of these animals that it would be easier for you to put them in a box and turn on the gas and not see what is happening, than to inject them one at a time. I guess there are a lot of people in this world who are able to perform these jobs, which are necessary in our world, without being traumatized by it, but I am glad I am not one of them.