This is just silly and pointless gatekeeping.
> [...] Only roles credited in the film’s legal billing and demonstrably performed by humans with their consent will be considered eligible. [...]
[edit] Now, the above is in the 2026 rules, but before this year, animals were still not nominated or awarded Oscars despite the rules not explicitly stating that animals are not eligible. Looking around, there's a lot of online discussion about this across the years. There are/were other award ceremonies for animal performers though, for example the PATSYs [2]. [/edit]
Now, why not an AI actor? For the same reason LLMs are not colleagues in the workplace. The anthropomorphization of AI has already gone far enough (case and point, the entirety of /r/myboyfriendisai on Reddit, or any discussion of personal and intimate relationships with AI), let's not legitimize it further. At best, it's special effects, not a cast member.
[1] See Rule 6, first section - https://www.oscars.org/sites/oscars/files/2026-05/99th_oscar...