We’re not sure if Barr thought we wouldn’t actually read the Mueller Report or if he was thinking at all when he stated that the president didn’t obstruct justice in his 4-page summary, a determination which as of today’s public release of the full report has been proven to be starkly different from what Mueller actually said.
Read page 220 of the full report. It clearly states that it is the job of Congress to ultimately decide whether or not the president obstructed justice. It’s called balance of powers.
See the full report here.