Hollywood. However, this isn't a new thing.
If a movie does well, or if it is based off an existing property, the likelihood that it will get a sequel is very high. Some sequels are simply pale imitations of their predecessors. 
Then, there are some sequels that take their source material and bring it to another level entirely. They expand the universes they are a part of, and make us all glad we got to spend more time with the characters.
With a new James Bond, "Hunger Games," and "Star Wars" on the way, here are some sequels that outdid their predecessors: