What does the launcher do with regard to the operation of the game that cannot be done within the actual game itself? Is it due to a technical limitation or simply there for the convenience of the players? Are there alternatives to such methods of starting the execution of games?
Luanti is kind of the obvious example to point to, with it being a community-developed engine for Minecraft-like games. But yeah, what @Azzu@lemm.ee said very much makes the difference. As opposed to Minecraft, Luanti has modding support built-in.