I don't think it should be surprising that there are (i presume) hundreds of old films with active social media accounts. If they have a presence / following, then making one small interaction keeps some level of awareness with fans. The film is still for sale, will still get streamed, will still be on TV. Studios have a vested interest in keeping interest in a film for long after release.
They will have a bunch of staff in their social media team who don't need to be paid crazy amounts of money, and that department will keep things ticking over on lots of accounts, and the overall long term result is worth the cost of running the accounts.
I have a friend who works as social media for a major streaming company and it amazes me how many people are in his department and just run twitter accounts. but the company has hundreds of accounts for all their products .