Why do organic conferences feel different than corporate ones?

Academic conferences, niche hobby cons, open-source meetups — there's a vibe in those that I can't reproduce at a corporate offsite no matter how nice the venue is. I think the operative variable is *what the attendees would do if there were no conference*. Open-source devs would still write open-source code; the conference is a side product of an existing community. Corporate offsites are constructed groups of people who otherwise wouldn't see each other. Implication: the "community" can't be designed top-down; it can only be observed and supported when it exists naturally.