First of all, it's really hard to replicate high-quality signals. Yes, we've got spammy guest postings and spammy comments and entire websites overrun with spam. But if you were to analyze those links, you'll notice that they are still islands. As in, do you see any Viagra-related links on Hacker News or in your Reddit subscriptions? Do you see Viagra-related links in your Twitter or Facebook stream? What about all your other news channels? The only places I see Viagra-stuff these days is either in my Gmail's Spam folder or on porn sites. And I don't know why, but I'm not seeing much spam in Google's search results while I'm logged in, maybe it's the stuff I search for, or maybe Google learned my interests - but in Incognito mode I get a lot more spam.
And second, if there are dark corners of Google's search engine, search keywords that have been overrun with spam, then Google is partly to blame because they've turned a blind eye towards spam for far too long, as they tolerated and still tolerate Adsense spam and content farms.
If Google is indeed facing a spam problem, then there's a whole lot more they could do. Off the top of my head, why not penalize websites hosted on old, insecure versions of Wordpress or Drupal? Why not expose a "Report Spam" button to logged in users? Why did it took so long for Google to detect and ban scraped websites?
SEO is sort of the same way in terms of there being a range of options. Some people might use SAPE, Xrummer or Fiverr or such (nude person with a URL streaking at a sporting event), whereas others might use more nuanced strategies where any SEO impact appears incidental.
Design itself can play a big roll in the perception of spam. Designs that look crisp can contain nonsensical content without feeling as spammy as they are.
But another factor here is that Google wants to keep raising the bar over time. Things that are white hat fade to gray then black as they become more popular & widespread. And as Google scrapes-n-displaces a wider array of content types, that forces publishers to go deeper (if they can do so without bankrupting themselves).