Edward O. answered 08/08/20
Film School Ph.D. Can Help You with Film Studies or Film Projects
There are many reasons.
In the past (and to some extent it's still true), studios must manufacture physical prints of movies. This is expensive, and so the studios only make as many prints as needed. Now in major cities, projection is digital, so there is no physical print. But this is not true around the world.
In addition, studios tend to want to build a film's reputation so that it is more desirable as time goes on. If they released a movie simultaneously around the world, there might be some curiosity, but no one could go knowing "it was a huge hit in the U.S."
Finally, movies are distributed and exhibited by different companies around the world. Each national market is somewhat distinct. French film distributors in general don't also distribute films in, say, Greece. So there is not just one marketplace with everyone buying at the same time.