When your shoulders and neck are tight, you tend to run with your shoulders hunched and your arms held out to the side. You probably also look like you have a short neck. Running like this keeps your arms from flowing back and forth, which affects your leg turnover, and simply wastes energy on holding your arms in an unnatural manner.
Perhaps you won’t be surprised by now to hear this is a form flaw we can pin on modern life. Hour upon hour in front of a computer or other screen tends to cause you to sit with your shoulders tight and hunched. Those postural habits tend to carry over to when it’s time to run. Good posture and ergonomics can help eliminate that chronic tightness, and simple mindfulness while you’re running can help relax your shoulders and neck until you get them to be so normally. Concentrate on having your wrists pass by your waist.