Between the constrained syntax, literal keywords, standard APIs and common coding conventions it seems like even independently-authored source code listings will be superficially similar. Certainly the basic/naive markov-chain-style logic that works really well for detecting even small examples of plagiarism in natural language content isn't going to as effective on `public static void main(String[] args)` or whatever.
Obviously there are strategies that could distinguish superficial/boilerplate stuff from truly duplicated code, but between the volume of actual (and often legitimate) duplication on the internet and the (maybe?) low ROI for duplicate content penalties for code relative to other value metrics/signals, maybe this just isn't terribly important as a quality factor?
ABC news, the NYT, WP and the WHO are considered high profile but the topic is not their area of expertise. Non of them would consider themselves the go-to place for people searching for it.
It's very possible. In general Google will penalize you for duplicate content, but that might not apply to code snippets since code is often re-used within projects or between projects.
The code snippet sites also typically have 2 or more snippets on the same page. When combined, it might then look unique to Google since their algorithm probably can't understand code as well as it understands natural text. Just a guess