That is because i feel like your supposed to ignore the boss going after the chests and kill the leader of the swarms as they are the "adventurers" with the semi rare to rare weapons.. the chest literally have no value except the odd seal or weapon. i've even seen some swarm leaders carry double drops. hence forth the longer it take the boss to collect items the better if you've got a tough enough team for it.. you've also got to remember from what we've seen there are no real super over powered weapons. take what you can get i guess