They just haven't sufficiently escalated the arms race. The "link juice" signals that g search has traditionally dealt with, and it's relevence measurements, etc., have been thoroughly gamed.

What they need to improve search quality is the intelligence to identify things as spammy and low value and then drop the rank of all the SEO optimized crap that people hate.

I can only imagine that google sees this as politically impossible ("you destroyed my business, Google! I'm gonna sue!") or they have decided the return on investment simply isn't there. They do not need to give you better results to keep getting search queries that help them to show you ads.