Cornell prof Tom Pepinsky nails it.
The implication of all this is that when we do find an independent variable that is clearly exogenous to the outcome we want to study, it’s very exciting. OMFG exogenous variation! But the fact that variation is exogenous has nothing to do with whether that variation is useful or interesting for the study of the outcomes that political scientists care about. Even less so for those of us who have a reason to care about a particular country.
…The problem, in other words, is that the search for exogenous variation has superseded the study of causal relations that cannot be cleanly identified. One of my colleagues relates the story of his grad school colleagues who instead of learning languages or reading history, “sat around in [BUILDING NAME REDACTED] trying to think up instruments.” Again, I am more than guilty of doing this myself, but when this trend dies down, we may be left with a discipline of political scientists who are unable to say anything about politics.