There’s also probably concern about proprietary technology as far as how you can mess with their code.
In any case, that’s only the problem with using assets from a game, you can’t use their designs either even if you make the content.
Nothing stopping you making Oiram Tark though 
The problem is in a lot of software we buy today, we don’t actually own the software. We simply purchased a license to use the software, and in order to use the software, we have to agree to their terms, whatever they may be. You may recognize this “End User License Agreement” that you have to accept before being allowed to install the software in question.