I'm confident enough in this take to write it as a PSA: playing music at medium-size-or-larger gatherings is a Chesterton's Fence situation.
It serves the very important function of reducing average conversation size: the louder the music, the more groups naturally split into smaller groups, as people on the far end develop a (usually unconscious) common knowledge that it's too much effort to keep participating in the big one and they can start a new conversation without being unduly disruptive.
If you've ever been at a party with no music where people gravitate towards a single (or handful of) group of 8+ people, you've experienced the failure mode that this solves: usually these conversations are then actually conversations of 2-3 people with 5-6 observers, which is usually unpleasant for the observers and does not facilitate close interactions that easily lead to getting to know people.
By making it hard to have bigger conversations, the music naturally produces smaller ones; you can modulate the volume to have the desired effect on a typical discussion size. Quiet music (e.g. at many dinner parties) makes it hard to have conversations bigger than ~4-5, which is already a big improvement. Medium-volume music (think many bars) facilitates easy conversations of 2-3. The extreme end of this is dance clubs, where very loud music (not coincidentally!) makes it impossible to maintain conversations bigger than 2.
I suspect that high-decoupler hosts are just not in the habit of thinking "it's a party, therefore I should put music on," or even actively think "music makes it harder to talk and hear each other, and after all isn't that the point of a party?" But it's a very well-established cultural practice to play music at large gatherings, so, per Chesterton's Fence, you need to understand what function it plays. The function it plays is to stop the party-destroying phenomenon of big group conversations.
I just ran a party where everyone was required to wear earplugs. I think this did effectively cap the max size of groups at 5 people, past which people tend to split into mini conversations. People say the initial silence feels a bit odd though. I'm definitely going to try this more