I've seen unions do many things that don't seem to make sense at first sight (and often not subsequently either). But in this case, it does make perfect sense to me - unions like SAG-AFTRA could claim that AI-generated "actors" used for speaking roles unfairly displace their own members while not paying union fees. So as part of their negotiations with the studios, they could demand that such "AI actors" would be registered in SAG-AFTRA as due paying actors [0].

[0] (For human actors) These are currently set at an initiation fee of $3,060, annual dues of $241.32 plus work dues of 1.575% of covered earnings up to $1M - https://www.sagaftra.org/membership-benefits/membership-cost...