It's the great workplace mystery: How does your slacker coworker who does almost nothing at the office still have a job when you know tons of great people who are out of work?There's a simple answer: Your manager is too lazy or too scared to do her job. And she's not alone--many managers allow their desire to be nice and to avoid difficult conversations to trump the fundamental obligations of their job. Among the most basic responsibilities of managing are holding a high bar and expecting people to meet it, warning them when they're falling short, and taking action when that doesn't change anything.[See 10 Signs You Have a Bad Boss.]Many bad managers, when faced with an employee who needs to be warned in clear terms that her job is on the line if she doesn't improve, instead take little or no action at all. Their reasons, weak as they are, are as follows:1. They can't bring themselves to have a hard conversation. Because it is hard. Telling someone that they're in danger of being fired is really difficult. It's also a manager's job, and they're paid to do it. Managers who don't are neglecting their most basic duties.2. They're worried about how much time it will take to find and hire a replacement. This is a terrible excuse, because strong performers will get up to speed quickly. But even if this weren't the case, a good manager would choose a short period of downtime followed by an all-out stellar performance over of years of mediocrity.3. They feel sorry for the employee. It's only human to feel compassion for someone who may be losing her job. Managers should feel compassion. But managers who handle the situation directly make sure that the employee knows what the problems are, knows that her job could be in jeopardy, and has a real chance to improve. That's far kinder than not being honest and instead putting her at the top of a layoff list one day.4. They're hoping the employee will leave on her own. This is a terrible approach; after all, if sales numbers were down, the manager wouldn't just rely on wishful thinking to get them back up. A good manager, when confronted with a problem, makes decisions and takes action--especially when it comes to something as crucial as having the right people on her team.[See 10 Signs Your Boss Just Isn't That Into You.]5. They're misunderstanding the law. Some managers think they can't fire anyone who's a member of a legally protected class (i.e., race, religion, national origin, pregnancy, etc.). This is a horrible misunderstanding of the law, which simply says that your reason for firing the person can't be her membership in that legally protected class. In other words, you can't fire someone for being pregnant, but you can fire someone who is pregnant for being a low performer.So if you have an awful coworker who somehow seems to hang onto her job despite poor performance, the blame lies squarely at the feet of your manager.Alison Green writes the popular Ask a Manager blog where she dispenses advice on career, job search, and management issues. She's also the author of Managing to Change the World: The Nonprofit Leader's Guide to Getting Results and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development. She now teaches other managers how to manage for results.