Because of hitscan falloff. Blizzard constantly nerf hitscan to the point that they cannot deal enough damage to Pharah before she hides behind cover. This mean that you can never kill her as a hitscan. If you go Widowmaker then the enemy will go Widowmaker so now you cannot focus Pharah or you will die to the enemy Widowmaker. Also the fact that Pharah has zero falloff so shooting a rocket halfway across map will still hit you for full damage.