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.

My guess is that people are really into the whole lore and world that they have a bunch of good ideas already. Fewer people already have a whole fictional world of their own built out. And there's a much larger audience for the existing world, that people wouldn't be as likely to give a new world a chance.

But I think it's mostly because people don't care too much at the beginning whether it'll be taken down eventually, they just have an idea they want to exist and put out there for other people to enjoy.

Part of it might be that the fun of it for them is creating it. Additionally it might just be a fun way to learn a certain technology.

A small part of me is interested in making an nes game, and the first step for me to learn that code would be to work on a fan project and improve existing code.

Sonic games are the right fan games to make.

2D Sonic games sucked after the Genesis (with the possible exception of the Sonic Advance series), but great fan games were being pumped out.

Some of the leads on those fan games got hired to make Sonic Mania, which is basically the Sonic sequel that Sega fans had been waiting for since the mid-90's.

Most recently I saw, portal 64 taken down by valve, you have to own portal, made for a dead old console.

Maybe edit the bits around so it goes through and say that your whatever is heavily influenced of X?