The most important element of the NFL draft is picking the right players. The hardest part of the NFL draft is picking the right players. Study after study suggest the ability to pick prospects is overwhelmed by randomness and variance.
Players can struggle to stay healthy. The coach who drafted a player might get fired and replaced by someone who runs a different scheme. College success might not translate to the professional level. Even if a general manager is great at scouting talent, he might get only a couple of drafts before he gets fired.