Key word being: unseen.
It was in the middle of a populated area, and he had no idea where the shooter would be.
Bullets are also smaller and faster. And closer.
Plus the Winter Soldier's arm isn't magnetic. Common misconception that Magneto controls metals. In fact, he only can control magnetic metals, hence cannot control Captain America's shield, Bucky's arm or even Iron man's suit.
It was in the middle of a populated area, and he had no idea where the shooter would be.
Bullets are also smaller and faster. And closer.