As companies gain more experience with the Open Source model, this will happen more frequently. It's something that we need to encourage for the Open Source model to continue to grow. But big companies still have a lot of issues with the Open Source model. First, it directly exposes the company's developers to the outside world. Corporate lawyers hate that.
Big companies tend to have a lot of money, and they can make attractive targets for lawsuits.
Even if an employee contributes Open Source code developed outside the company on the employee's own time, the company could become a lawsuit target if that code violates a patent. Further, rightly or wrongly, companies have information that they consider secret. They worry that exposing internal employees directly to the outside world also exposes their intellectual property. The Cathedral model often makes it hard to contribute to Open Source projects, because the leaps in development are often too large for developers to digest. This makes enhancements or new projects likely to get rejected for the simple reason that they are too big and too radical.