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.
This works by replacing assets, config files or even by injecting binaries into pre-compiled games, as well.
Minecraft is in a pre-compiled language (Java), but so is Skyrim (C, C++) which is famously modded (although most mods are written in Papyrus which is an interpreted scripting language, the game itself wasn’t).
Most mods for Tekken (C++) also work this way; they simply replace assets and config files.