Two possible reasons:
- Risk assessment: In 2 - 5 years, will there still be further development, bug fixes, especially of security bugs. Taking over responsibility for all those abandoned projects is not an option.
- Cultural differences: Large corporations understand requirements of other large corporations better. Like long-term maintenance commitments.
Legal departments in large consultancy companies have been working on "open source policies" internally (mostly on the "using open source software" angle, less on the "contributing" angle).
And those policies are specific/hard/complex enough to discourage public endorsement.
For instance, use of GPL-based software is strictly forbidden (for good reasons, when you take the perspective of the company); unless a specific review by legal allows it, in which case, you've already been through several layers of management with a convincing case for each of them.