Most golfers cannot reach a parallel position at the top of their swing without collapsing the arc and getting narrow.
The usual culprit is the bending of the left arm at the elbow as they approach the top, because they simply do not have enough flexibility throughout the back and neck area. Another root cause of a narrow position is from not making a large enough shoulder turn.
Many golfers lose a ton of distance by not cocking the wrists enough.
While it is unnecessary to achieve 120 degrees of shoulder turn like Dustin Johnson to hit the ball farther, we should strive to get 90 degrees out of our windup to the top of the golf swing without losing the original measurement from the base of the neck to the club handle.
Additionally, once we get narrow we lose the collision course with the ball as we are on a different ‘track’. We will need to find the track again before we get to impact. This makes striking a pure golf shot a bit more complicated.
The good news is that this a skill that can be improved tremendously at any age. With the right tools, and working at it every day, you can widen your backswing and increase your clubhead speed while using less effort.