Generally (especially as companies grow) it's all about liability protection. Companies don't want to get sued, and they don't want to lose if they do get sued. That means paper trails, and attempts at showing a history of objective criteria leading to the decision, etc. This often looks like performance improvement plans and so forth, and so it can be pretty easy for someone to do just enough to not get fired but still be a quite low performer (especially with inexperienced managers).
This is flipped on its head for things like abuse where the company could be liable for not responding to reports in a timely manner. Cross a line behavior-wise, and you can get fired quickly.
Large organizations (and many small ones) just don't do it if there's any way to avoid it. They're rich targets for lawsuits, of course, but more importantly, it has widespread effects on morale. Fire one guy, and ten start thinking that their jobs are nowhere near as reliable as they had thought.
What's hard about firing people? I actually don't know and would love to find out if you could share your thought