Eventually someone was going to create an easy to list/share/subscribe list that individuals could easily add to their personal Google domain block list. Think EasyList.
At that point they would be bleeding ad revenue as all the nasty, fake, abusive, spammy websites would be insta blocked.
Imagine being able to add a list and all of a sudden half the SEO blogs are excluded from results. Assuming Google even allows it, they would then have to work even harder to find relevant content to your search query. They can't rely on throwing a huge wall of semi-relevant results that you have to wade through, generating ad impressions as you go along.
