Why are there currently no content-based evaluation metrics for information retrieval?
Using a word-based metric would explicitly favor word-level retrieval methods.  The theory is that (just as you suggest with dwell time), the URL-level metric measures more directly the desired result.  

More concretely, consider a search of "alcohol from potatoes." Assume we have two pages: 

1) A page that is simply a grocery list (containing "alcohol" and "potatoes") 
2) A page that describes vodka or moonshine which only uses the words "ethanol"
   and "potatoes" but not "alcohol."


A word-level metric would prefer the first result, since it contains both alcohol and potatoes..  A URL-level metric would likely label the second result as more relevant, which would allow a model to form associations beyond simple word-matching (i.e. it could learn that "alcohol" and "vodka" are associated).