Because if they do, they're legally liable for facilitating IP infringement which could cost them substantially more money than Palworld could possibly make. Same reason steam didn't allow the Dolphin emulator to get a steam release. Cynicism towards the gaming industry is a good thing, but you still have to apply logic and follow the money.
The AI claims are coming from ******** Pokémon fans because the last three Pokémon generations have been unoptimized trash and that isn't going to change anytime soon because as someone else pointed out, the games are no longer the focal point of the brand. They found a quote from one of devs saying he was interested in the possibilities of AI in gaming and ran with it. There isn't a publisher in the industry currently save for maybe devolver digital that isn't excited for AI asset generation. Squeenix has an entire vault of 30+ years of JRPG concept art that they own that could help them cut dev time down by at least two to three years with the aid of AI. This is the distopian future we live in unfortunately.
What bothers me is when fandom gets in the way of facts which is how these AI allegations in Palworld came to be.