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?
That’s because it’s easier.
Most games do not even have official modding support, Minecraft included. Most are only moddable because they use some interpreted programming language somewhere in their game, which people found ways to read and inject with their own code.