They do so for the same reason they did that before SO and SE existed. A good chunk of people like to help each other. Another (partially overlapping) chunk likes to have their knowledge challenged, another (also partially overlapping) like to show off their knowledge in front of their peers. This was what made the Internet tick since the earliest days.
SO happened to come from within the community (Jeff and Joel were known experts), and had the right features to eventually channel most of the drive I described above through itself. But it's not like SO created it. SO is also accumulating some anti-features these days (like well-known propensity for closing 90% of interesting questions as duplicates or off-topic, or the recent moderator dramas, or targeted advertising they snuck in), so their future is somewhat uncertain.
> Again, lot of these volunteers are highly paid IT/tech folks, not your ordinary 8chan Joe.
A lot of "8chan Joes" are highly paid IT/tech folks. Or at least I'd assume so, given that it's true for 4chan.
> They could have been doing anything else in their free time instead of technical answering questions at SO. (...) Again, no one gets paid for anything they do on SO.
People also answer SO question on their employeers' time.
> This is the beauty of gamification and extremely well designed community driven completely decentralized governance at its extreme.
SO wasn't a "big design up front". It evolved over time. Gamification helps a bit, but I question it's critical to get highly-trained professionals to answer complex technical questions for free. Again, a lot of highly-trained professionals do that simply because they like it. On SO, or on HN (one of the big points of this site), or on old-school forums, or on conferences, etc. etd.
Even if I were 100% wrong, this would be offputting. But I don't think I am. I'm in the top 100 users here; I was a Quora Top Writer; I was a Wikipedia admin; I've worked on community sites, including a Webby winner. I know something about online contribution. Yes, SO's design got them where they are. But that doesn't mean it's perfect. And until SO's fans can start seeing those issues, they stand no chance of getting fixed.
The answer lies in its magnificent design. SO has community based voting for almost everything. Almost everything is gamified. There are people literally writing essays on why everyone should vote them to be an "officer" for X and how they will work hard on keeping quality bar high. There is an extra-ordinary passion in what they do. Its HN on steroids but with much better mechanics. It is almost completely decentralized and operates without needing people blessed with special dictatorial powers 99.9% of the times. Again, lot of these volunteers are highly paid IT/tech folks, not your ordinary 8chan Joe. They could have been doing anything else in their free time instead of technical answering questions at SO. This is much harder than dishing out fact-free opinions on non-technical websites as armchair expert. Again, no one gets paid for anything they do on SO. This is the beauty of gamification and extremely well designed community driven completely decentralized governance at its extreme. Its almost magical to me and I haven't found any other website on Internet achieve it at same scale and importance.