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?
Not sure about Minecraft, but sometimes the launcher is simply there to make configuration, startup options, parameters, profile selection, etc. a smoother experience for the user. You can do this kind of stuff without a dedicated launcher, but some software publishers prefer to divide things up this way.
It’s a dumb analogy, but I kind of think of it like restaurants. Some restaurants have a full wait staff with a maitre d, some have walk-up counter, some have a kiosk with a screen, etc.