Because games are built with engines, and you're not going to re-implement all the simulation / systems in a different engine or language. Why would you? A game

Because games are built with engines, and you're not going to re-implement all the simulation / systems in a different engine or language. Why would you? A gameserver is basically a game client stripped from rendering with a bit more logic. I'm talking real time games here, not an .io game over websocket/json.