Even if they wanted to open source or just share the source, there could be some real effort involved in reviewing the code to ensure that nothing sensitive will be exposed.

Re the other comment about owning copyright on everything (eg. Due to components being licensed from other vendors), they could rip that out. Thinking about this triggered a memory of when Descent 1 source was published, they excluded the sound library for this reason. Could be tough to excise something more integral though I guess.