It takes time to select, communicate with, and manage an outside company. It creates a dependency on that company's schedule and the schedules of all the other companys that the outside company also works for. Those costs tend to be non-trivial even in cases where there is a long term established relationship.

To put it in perspective, it may take a couple of weeks of calendar time just to get to a signed agreement. Never mind a few more weeks to get a final product [if it all]. That's a big break in cognitive flow for a small project that can be knocked out in a few hours.