Of course the administration uses cherry-picked statistics to get credit. And of course the opponents of the administration use cherry-picked statistics to criticize the administration.

Beyond obvious short-term political goals, the reason for this is simple: in a machine as large and complicated as the government, it's really really hard to accurately ascribe meaningful credit (or guilt) for any change, and especially to do so in a way that is understandable by voters.

Each administration makes changes (positive and negative) that won't be felt by voters for decades, if at all; and under each administration, voters endure the consequences (positive and negative) of previous administrations. But that doesn't get people to the polls, so why talk about it.