I find the fact you mention Wikipedia and Pinterest in the same breath surprising. As someone who doesn't really use the latter, I always knew it as the site to exclude from search results. Whatever the query, a result in Pinterest will look like utter spam that only used my query to get into SERP and then try to make me go to something barely related. On the other hand, Wikipedia seems to be always on topic and usually high-quality content. I understand that one medium is more vulnerable than the other. But still, with somewhat similar content creation structure, I'd expect more similar outcomes. So, what is the main reason for the perceived quality difference?