These runners run fast enough that much like cyclists, they get a benefit from following in the aerodynamic drag of another competitor. Unless there is a pace

These runners run fast enough that much like cyclists, they get a benefit from following in the aerodynamic drag of another competitor. Unless there is a pacer none of the runners really want to be in front and eating all the aerodynamic drag. So the race slows down. This greatly favours runners with a faster sprint at the end.