Because it leaves a better impression than if you were fired after just a few months. However, if you did something really bad (e.g. showing up wasted), it would not leave a better impression no matter how long you stayed. That's an exception.

My coworker from McDonald's where I worked back in 2009 been canned for her outside life interfering with work. She had since been rehired. Another coworker at Walmart been fired bc a manager didn't like him for no good reason. He has also been rehired. I'm back at McDonald's after 8 long years bc my records said I quit and nothing else.