As someone who has done this, I can vouch that's not always the case. Some do it because they're tied up with something that happened in the house. Others do so out of insecurity and fear that they'll be ridiculed depending on how they play.
I’m talking about when no one is playing or talking or anything, just a bunch of people sitting in a lobby doing absolutely nothing
They're probably too lazy to turn the console off, or they're so caught up in private chat that they forgot they were in the lobby.