Why don't papers cite results within their references more specifically?
In the introduction section of a paper, such specificity might be considered out of place, since the goal is to provide an overview of what has been done without getting too bogged down in particular details. 

In the methodology or main section of a paper (depending on the field), such references might be more appropriate, and I have seen papers that specify individual equations, tables, or figures. There's no reason they couldn't do so; I don't believe that there is any stylistic justification that would preclude it.