Because melee is a more vulnerable position. While you are threatening your enemy with OAs, the enemy also threatens you with them as well so you're in just as much of a situation as your opponent. Being in melee means that you're likely not in cover from other ranged attackers in the enemy ranks. You also can't go prone against your ranged enemies since a melee enemy will then be more dangerous.Did I miss the context? Why, exactly, should melee be rewarded over ranged combat
With all these concerns, you need to have some good incentives which melee provides through its other good facets.