This really varies per site. For example, since StackOverflow is primarily for concrete technical questions, overly subjective discussion questions are liable to get moved or closed; however, if you're interested in something concrete but still broad (e.g. how to use existential types), you're likely to be fine.
On programmers, on the other hand, most of the questions involve a good bit of discussion; the majority have several answers. This is because the focus of the site is different.
Finally, I think there's a lot of selection bias--you're more likely to remember closed questions, and the sort of questions that get shared on HN are the sort that don't really fit in StackOverflow.
On a mostly technical note, you're most likely seeing closed questions, not locked ones.
Locking prevents discussion, answering, or any other actions by anyone except one of the 12 moderators; there are currently 16 locked questions on the site.
Closing, on the other hand, can be undone by any five of the 7500+ users who have 3000 reputation.
StackOverflow is not a site dedicated to hosting discussions on "interesting" topics. It is a site dedicated to getting the highest quality answers to technical questions. And it does that marvelously. As a byproduct it also fosters the sort of community that ends up providing interesting discussion from time to time, but if the site became about that it would not last and it could easily cause a derailing of the site's main purpose.
Just because something is "good" doesn't mean that it belongs on SO.com.
(1) A single question (as you say)
(2) (Attached to the question) Some whiny git putting on his internet cop hat and suggesting that the question is inappropriate
(3) An almost immediate locking of the question by said whiny git
(4) An interesting discussion
Almost every time I see some moderator putting on his 'big boy pants' I want to reach down the internet and slap them.
I just wish they could find a way to send the gnomes back to wikipedia.