My guess would be that people have a pretty good idea about what's plagiarism and what's not. As some monks pointed out, there can be a fairly long period between the time a question is first read, and when the answer gets posted. It's not always plagiarism if the answers are similar.

I use down-votes sparingly, for the most egregious "sins" (such as not bothering to try to figure it out before posting a question). Only a word-for-word repeat of a post would convince me of plagiarism, because many coding issues do have a single optimum solution.