A lot of people posting stuff which is not wrong but im going to post another smaller piece here as well. People who go into the job are a particular type of personality and the hiring and training process further weeds out the unlike kind. But then on top of all of that they tend to only see the very worst in our society as well. Which then as the ones who survive begin training the new the training gets more skewed to protecting against that worst kind. Its a negative feedback loop.

That is not to say what others have said is wrong. Its just not one specific thing and the type of person able to survive in that job without being killed or killing themselves does not garner the type of person we as a society would like to have working that job.