Google can properly classify email spam.

They should be able to fight this as well.

Problem is they are the ones funding the poor quality spam and they're in turn profiting by taking money from advertisers.

Google used to penalize websites for SEO hacks. Then they started attending SEO conferences themselves. SEO specialists didn't outsmart Google, Google stopped trying. As the sibling implies this is most likely because they noticed that the easiest way for SEO spam to monetize is ... Google ads.