Why do so many large companies keep an internal IT department rather than out sourcing?

IT departments are a service organization. When they are saffed internally the service is usually pretty bad. Why not outsouce with customer satisfaction ((i.e.employees) being a major component in receiving/renewing a contract?
Update: Confidential information and security access is a concern with any contracted service yet those are out sourced any way. Governments out source all sorts of security operations yet breaches are rare.
Cost is usually cheaper and more easily controlled with contractors which is why they are used for services.
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