Fair use still protects Intellectual Property. You don’t own the code for a game, if the game company asserts their rights in the documentation (and they do usually).
Now, when it comes to things like games, songs, movies, etc - there is some not-for profit fair use for personal or educational reasons. Blizz is not going to come to your house and pester you for what you do on just your PC for your own use. Just like Disney is not going to get upset at someone drawing a cute version of one of their characters for personal use. That does not extend to distribution of those materials - even for free. Blizz is very very very picky about that - they don’t want you modifying their Intellectual Property and distributing it.
Look up DOTA if you want to have an idea why.
Rather than deal with any of that though, they just keep the code locked down server side and there are no issues to speak of. People can’t mod and distribute a game they don’t have access to. Reverse engineering is very much against the law (for currently protected IPs) and they go after that such as the illegal WoW servers.
If people really want to mod and create their own visions of games, there are free or inexpensive game engines out there that they can learn on. Create away! Give it away for free if you want! It is YOURS using a platform you licensed and paid for along with art and concept assets that are yours.
I will never understand why people would even think they are ENTITLED to modify something they don’t own the rights to using the engine and art assets they did not create. If the game maker allows it great, if not, we are not somehow entitled to it.