Just think how you feel when you are at a meeting and two key people start whispering to each other in a side conversation. Of course not, you are insulted, and rightfully so. Expand that to the way all information is handled in the company and you have a culture of secrecy. That may sound obvious, but I regularly run into client situation where people start whispering to each other. The speed of the demand for information and the frequent change of the content have added to the difficulty.
Formal databases are one solution companies are trying with mixed success.
But to have a culture of information sharing, you have to have the exchanges happening informally among people in the company using every tool available. And then listen to each other's answers. In many traditional cultures, it is a sign of weakness to ask that question. That has to change. Reward and incentivize both the sharers and the askers.