If jobs are so scarce then why....

Okay, I'm sorry I am going to rant about this a moment. I know jobs are scarce right now and I think it really sucks that good people can't get good jobs. When thousands of people stand in line trying to get one of only 35 firefighter jobs available, you know it's tough. I'm sure most of them were qualified too -but just couldn't find work.
But here's a question....with so many people needing jobs, why is it that some companies feel the need to keep incompetent workers?? I mean have you ever been to a store and the store clerk just didn't know their job or wasn't able to help you at all or even worse yet, were rude and unhelpful?! I am all for on the job training and I will give anyone a chance to learn and correct their mistakes, but I personally think that in an economy like what we have now and with the unemployment rates as high as they are...wouldn't that just make you want to do better and work harder?! It does me!
I'll give you an example....a silly one, but one none-the-less....I drove through McDonalds the other day to grab food for my family. As part of my order, I asked for 2 chicken nuggets kids meals, one with Dr. Pepper and one with Apple Juice. I also asked for a Quarter Pounder combo w/no onions and an Extra Large Iced Tea. First the guy said they couldn't give me an extra large - I told them yes, he could (it would just cost a little more) and that they do it all the time. (I know as my husband likes the extra large iced tea.) Then he said okay, he could do it. I noticed he put on the order screen that I wanted a chocolate milk on one of the kids' meals and that I wanted extra onions on the Quarter Pounder. I corrected him and said I wanted a Dr. Pepper and NO onions. He said, "Yah, yah". I said I want to ensure it is corrected because I don't want to get up to the window and be handed the wrong order. He replied, "Yes, yes, it will be correct."
Well.....can you guess what happened? Yes, my order was wrong. I got up to the window and they handed me an apple juice and a chocolate milk and were about to give me the burger with extra onions. I said sorry, that's not what I ordered. I then asked if I could speak to the manager and told her what happened. She apologized and they worked on getting me what I had actually ordered. (So not only did the guy not correct what he said he would, but he caused his co-workers to have to re-do the order they had already prepared.) The manager and other worker were friendly, quickly corrected the order and apologized for the other employee.
But here's what I don't understand....why keep someone like that? Why keep someone who was rude and created extra work for his fellow employees. I could understand if it was his first time and he corrected his mistake, but based on his attitude, he didn't seem to care. I would think he would care....with all the people needing jobs out there, why should he be kept?
It's something for people to think about. I would think people would want to learn fast and do a great job, no matter what the job. I sure do! Sorry....just had a rant a moment there.