The real question is why does Google still put them on the first page? They've dispatched other nuisance sites but Quora still seems to get special treatment. Quora can't possibly have many inbound links to their content so why the high page rank?
Look at how long pinterest results have ranked highly in image search, even though they're completely useless. I just don't think Google cares to remove them, since surely they have known that pinterest should be excluded from image search results.
Google has some strange preferences. Forbes still dominates SERPs despite one of the worst user-experiences of any mainstream site. It's downright unusable without ad block
IRL social networks. The guy who wrote the Quora algos probably has friends that work at Google who can influence the pagerank. I've seen these kinds of interactions first hand at various events in Seattle-Bellevue around 2014-2016. Only recently has it died down (mainly because the meetup groups either fizzled out or stopped offering free beer).
