Is that a serious question?
What use is the source code to a normal user? They do not have the knowledge, nor the tools to create the game from the source code. And even if they have some of it, chances are the game might not work, because of slight oversights or technicalities.
Users are here for the game, not the source code.
And this is not even considering that some developers might not want to share their code.
It is also kinda the name giver for open source. Everything not open source could be called closed source. And you do not share code from closed source in general. So it is unusual to see source code outside of open source projects.