Right, I understand your point, and maybe I'm being too reductive here, but the idea heuristic that what people link to is authoritative is an opinionated decision that ended up being satisfactorily accurate (and efficient to implement). And evaluating the heuristic's accuracy ultimately boils down to human judgment; from their early Google proposal (though not the original Pagerank paper):

http://infolab.stanford.edu/~backrub/google.html

> The biggest problem facing users of web search engines today is the quality of the results they get back. While the results are often amusing and expand users' horizons, they are often frustrating and consume precious time. For example, the top result for a search for "Bill Clinton" on one of the most popular commercial search engines was the Bill Clinton Joke of the Day: April 14, 1997. Google is designed to provide higher quality search so as the Web continues to grow rapidly, information can be found easily.

(Yes, the opinion that a "bill clinton" query should probably return official/biographical pages before "Joke of the Day" is a pretty obvious and uncontroversial one)