Because IT is your social media's business. They know IT inside and out. They understand what can go wrong and how to mitigate it. The business for airlines (for example) is to fly planes. They are pretty damn good at it. IT, however, is just a tool to them that they buy elsewhere. They don't understand it in the same way as social media. They rely on outside contractors to do it right: outside contractors who get the job based on being the cheapest or convincing the buyers their service is "industry best practice."