Let me put it this way: mobs you AoE are redshirts from a Star Trek episode, while the bosses are actual named opponents from the episode. You can mow down redshirts by the dozens and nobody would bat an eye, but unless the boss is a jobber you wouldn't be able to kill him that easily. It's the same formula across the board for multiple genres, video games, literature, or movies. You never see Picard/Luke/Dynasty Warrior hero of choice/Link/Name your own hero spending an entire episode/game/book dealing with one measly peon. Why? Because it's not epic, and it's boring as hell.