Public sector here, can speak to many of the issues as to why low performers are more or less ignored and bosses let bad situations play out.

After three years of service you are tenured, or become a permanent employee. You can read about the wide range of protections that permanent employees enjoy, but it essentially boils down to not being able to be removed from your job unless a number of things happen.

It's been said on here before but bosses just don't have time to deal with poor performers. Instead of nipping the problem in the bud they tend to let poor performers coast, focusing on their good employees instead.

Here's where the boss and the system eventually fail. If you give one or several positive or neutral evaluations to a poor performer they now have paperwork documenting that the boss views their performance and work as satisfactory or exceeding expectations. A lot of times bosses will do this because they don't want to get hit by an EEO complaint or they don't want to turn a poor performer into a malcontent, someone who will negatively shakeup the office dynamics/chemistry. Once that employee has that documentation saying that the boss views them as satisfactory, they've essentially got a free pass to do whatever they want for the next quarter or half year.

I recently left a job where my supervisor was a good guy but a very weak leader who had no will to execute discipline, he was a pushover. He would issue these OK/satisfactory work evaluations and when an employee either refused to work, didn't show up on time, or was causing trouble, he was hit by an EEO complaint. The problem got so bad, and he had acquired so many complaints that management had to send poor performers to other work sites because they were useless at HQ, so they dumped them off on someone else. These employees couldn't be fired because they had tenure, and they had received positive evaluations in the past.

I hope I was able to answer some questions.