Because any kills they get ahead of you are kills you have no chance of getting a drop from.
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That is true, but seeing as they rush through, you will have more enemies left over anyway. Maybe not the final ads with the boss, but along the way for sure, unless there is one of those kill them all before proceeding portion. Those suck and it pays off to be there when it happens...
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He basically wants the drops from their efforts, and if they aren't killing to speed up his farming, he's not happy about it. Otherwise what you said would be true, but he wants to use the rest of the fireteam to make his own mobile loot cave.