Tonton Streaming Seconds from Disaster (2005) S1E12 - Collision on the Runway

Two Jumbo Jets, Pan Am 1736 and KLM 4805 collide on the lone runway at Tenerife, Spain. The two aircraft are then burnt out and 583 people lose their lives. But how come two experienced pilots made a mistake of this magnitude? How come two passenger jets collided on the runway?