I would point you to Netflix, a large company with a decent culture. They didn't ask every employee for input (AFAIK). They hired one person and gave her the time and resources and access to upper management and tasked her with coming up with the culture document. 12K people in a stadium for an hour or even 4 hours won't be productive like you want it to be. In the end only a few people will speak up because not everyone speaks up in a stadium full of people.