Are they? Google could manually remove the worst SEO sites from the index and like reduce the spam you get for dev queries with like 95%.
It takes time to build up rank and like 10s to remove the site.
There is an extension that does this client side for Firefox. Try it. (I don't remember the name. I only have it on desktop).
The only explaintions are that Google is too dysfunctional to do anything or they are doing it on purpose to increase revenue. Otherwise there is no way the SEO would suddenly become this bad.