Why Can't We Pay Fanwork Creators?

I've always wanted to be a writer. My mother was a journalism major, so I've been exposed to writing my entire life. I filled notebooks with my own stories and, eventually, my own fanfiction once I got into Star Wars and Pokémon. While I don't think my dream of being a writer is truly attainable for me, writing stuff on my website is about as close as I'm going to get.

All that preamble to say, I have a lot of respect for writers. This brings me to a question I've had for a while: why are people so terrified at the idea of giving money to fanfiction authors?

Among artists, donation services like Patreon or Ko-fi are common. Sales of merchandise and other physical goods are also frequent. These things remain true even when the art is based on copyrighted characters and works. And yet, among writers, these things are shunned and vilified. These things are even explicitly banned in the Archive of Our Own ToS.

Fanfiction isn't the only form of fanwork affected by this. In the Pokémon fangame community, people get extremely upset and defensive when someone tries asking for donations for their game, even if a large amount of custom content and work was put into it.

So, why can't we just treat all fanworks equally and let creators receive a tip for the work that they do?