Those creators are– most of them knowingly– putting themselves at risk legally.

When you post fanworks on AO3, if a rights holder like Disney tries to take legal action against you, AO3 and their parent organization the OTW will go to bat for you with their lawyers and legal fund.

When you post fanworks on Patreon or Tumblr, if a rights holder like Disney tries to take legal action against you, Patreon or Tumblr will happily hand over all your information to that company and you are on your own legally.