Film studios are constrained by labyrinthine contracts between studios, distributors, actors, and unions. In cases where contracts are missing, any of the above parties can sue.

Even more confusingly rights for IP might have been transferred and sold many times leading to questions about who actually owns a film or franchise.

Distribution rights, production rights are separate. Actors may have been promised a cut of any of the above.

So if you track down 40 years worth of contracts, you can safely add a movie to your streaming service.

Add to that 100 years of studio mergers, consolidation, etc. It is a mess.

I am sure many companies would like to stream more, but simply can't.