First at all you could stay back and kill all the monsters if you want, they aren't stopping you. Second, have in mind that many of these people is trying to finish the Bad Juju bounty which requires you to complete 25 strikes, in order to do this you don't have to kill all the monsters, it gets boring, trust me. And finally, before the Cryptarch patch it was worth to kill every single monster and then the boss (because of higher chances of drop) however this patch turned this around; you get low chances of legendary and rare engrams dropping but you are going to get an rare/legendary engram (for sure) plus random items (from rare to exotics). So think about it.