Why don't papers cite results within their references more specifically? For example,

An explicit formula for ___ has been derived in [1].
[1]: Alice and Bob, Journal of Academia StackExchange, Vol 1 Iss 1, 2016

Problem is, the paper cited could very well include lots and lots of formulas. The desired formula might just be one of many results presented, and it might not be the central one.
Why don't authors write the reference as Alice and Bob, ... 2016, Equation 12? Even just giving the section name helps identify the relevant part of the reference, and it'd be a great time saver. However, I don't think I've ever seen anyone try this in formal publications. Why?
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.