Honestly it's because in party chat you can focus on the party chat more then the game noise. If the game let you turn down the "pew pew" sound effects so you could more easily hear party members it'd be no big deal. I have a fancy headset so I can do that in game but a lot of people can't.
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I can do this with my stock in-the-box headset. Pew pew noises controlled through my AV receiver like normal, then game chat volume through the buttons on the connector attached to my controller.