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?

They are just different mindset.