People like a creative work and are creative. Do the math. Just like every cyber security person started with hacking, every creative starts with derivative works.
Portal 64 was taken down by Valve only because it used Nintendos N64 SDK which was not publicly released. Valve is cool with fanworks for the most part, even allowing them to be sold for profit (Black Mesa). Nintendo (as with most Japanese game companies) are the worst offenders of the extreme opposite.