I've done this gathering requirements, getting acceptance (PR, release notes, tech docs, UI designs), problem solving, etc. Make up something plausible, but wrong, and label it "final proof" with a deadline two days away. This technique works a little bit like adding a duck (Battle Chess reference).

Works for me because I care more about winning than being right, so I'm not embarrassed to be wrong.

Lot's of reasons people withhold information for whatever reason. Most geeks are introverts, so are reluctant to speak up. Some people don't want to be embarrassed. Many, many aren't aware of what they know. It's easier to criticize than to create. And, yes, some people are gate keepers, hoarding information.