How come a company who does X and already have employees doing X can't find new hires?

I'm sincerely wondering. So if you have 50 people on the payroll to do "SAP", where did they come from? A school? A course? Didn't they have coursemates to reach out to for more workers? Don't people and companies have networks? How can things deteriorate to the level that you have to put out ads for total strangers to apply?