To put this in a slightly different perspective. Let's say you had hired a babysitter so you could go out for the evening. At the last minute, the babysitter cancels, and it is too late to get another one. Now you are either scrambling (calling relatives, etc) or you cancel your evening plans. Now let's add to this that your kids really like this particular babysitter, who has quit the babysitting business permanently, and it takes a bit of trial and error to find a replacement that will work for your kids.

So an employee leaving does have some impact. On the employee side, of course there is the food-on-the-table issue, which can be important if there isn't a safety net. Which is why it is critical to live within 75% of your paycheck, so that every year you can survive 3 months unemployment.