I can't prove it, but if bet anything the reason Google is getting worse is the same reason everything gets worse: engagement. They are likey optimizing results not for relevance but for engagement more and more.
Not just that but bias, they intend to promote certain content and demote others and that introduction of human bias essentially dillutes the effectiveness of the algorithm; you don't get what you are searching for per-se but someones opinion of what you should get.
