Recruiting, benefits, taxes, time-off, payroll, office space, computers, personnel issues, training, expenses, management. They're all very real aspects of having an in-house ad operations team. Many publishers find it very difficult to grow or even replace ad operations people let alone hire their first AdOps person.
Do you really want to be dedicating so much valuable energy and resources when over time you could be putting it towards project and tasks that grow revenue?