Because the other team has earned the right to stomp that team out of existence. Just because a few bums leave early, or get disconnected, doesn't mean you get to start with something that can shift the outcome in your favor even though you've come in late against your own choice. Take your ass pounding, grab the rewards you still get regardless of the outcome, and hope you get a fresh match in a few minutes. If winning mattered, maybe there could be a different discussion.
What I think is a bigger travesty is Bungie errors removing you from an activity when you had full special/heavy and then when you renitence you're back to zero. That is truly -blam!-.
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Those "Bungie errors", at least in Crucible/Gambit, are often the result of network manipulation by cheaters on the opposing team. I've had many instances of being temporarily black screened during a match right after popping my Super, or just as I grabbed or am about to grab heavy, and when I reconnect to the match, my Super is gone, I have no heavy, and my score is wiped clean, as if I just joined the match.
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Happened to me during a gambit match. Couldn't tell which one it was that booted me.