Some companies like to be cheap and not hire sufficient number of engineers. Companies are fine understanding redundancy for their hardware and their services etc... They totally failed to grasp it for their employees. That's all fine and good if you're putting in a resignation and giving two weeks or a month notice but often times things are not planned. Family emergencies happen where you have extended times people can be out, grave injuries or death happens or people can just be gone the next day.
So however you plan for those eventualities is the same way you plan for and manage the knowledge transfer on resignation.