What enrages me the most is that Viacom cares enough for MLaaTR to copyright strike people, yet they can't be bothered to even air re-runs in Nickelodeon or even add it to Paramount+.
These people are the worst.

It comes from the assumption that the company failed to see the derivative work as a parody fandmade material, instead deeming it close enough to the original work to copyright-strike it for infringement. Nintendo is known to operate this way for their IPs.

The only other reason is when a fanmade work so just happens to be something the company has planned to do themselves, such as what happened with that Mario Battle Royale game or the Metroid game which, funnily enough, also leads to the "porn is canon" route somehow.