I think in the Discord model it makes more sense because it's based off of how online gamers flow (basically open a voice chat and stay on there for hours on end).
Gamers obviously don't talk only when they open discord. They talks in 'game' with discord minimized. It would be dumb if hide discord to background while keep chatting is a manual action by default. But that behaviour can be changed. You can ask discord to exit the app when you click x if you prefer. There is an option for it.
And about teams... who the heck want to be kept in a meeting if they are already ready to close the app?
Closing the window frees up some video RAM, as you are no longer processing and displaying video streams from the voice/video chat. Minimizing the window keeps that RAM in active use.
Keeping the meeting going in the background is honestly pretty crazy.