SEO is supposed to be StackOverflow's core competency. They are completely aware most people end up on their site via Google. The search on their own site sucks.

The reason Q&A sites are so visible is that people tend to type questions in their search engines, so Q&A sites are a good match to those.

Not really, a big website can not cover all keywords in its niche, no matter how big it is. The strategy for small sites is to focus on long tail keywords (3-4 terms) and outrank the big guys.