It would waste battery on mobile devices if this was widely used.
The Multiplayer plugin is the exception - it can keep running if you are the game host, since otherwise it would kill the game for everyone connected - but apart from that, it's probably just obnoxious to the user to have a background tab keep consuming CPU and battery power.
I use my own design java websocket for my big online game
i make it about half years
I hope I can set background running myself