I think what you will see (by early 2022), Amazon will market two additional video-streaming services...one for the 1930s/1940s/1950s, and one for 1960s and on.
If you gaze over the catalog of MGM materials, just for the 1940s...it’s a five-star listing.
MGM was busted up as a monopoly in the 1960s.
The game may have changed in the 70s even without the government interference but those type of films stopped being made (and MGM sold off their prop and wardrobe collection that made it cheaper to produce movies because you went and used what you have made for some other movie).
Even Roger Corman used another studio’s old sets when he made Masque of the Red Death in England (with Nicholas Roeg).