When Disney bought LucasFilm to get their hands on the Star Wars franchise, it became obviously clear that they were doing this because they wanted to create further movies. After all, they were unlikely to recoup that $4 billion by just working with the existing material and property. To make money they would have to set about creating a whole new series of films and licensed products that would entrance fans and make them flock to the cinema.
Sequels are an integral part of the gaming industry. Just like with other entertainment mediums, successful titles are seen as perfect examples for further exploiting. It makes sense, as fans who have already enjoyed a particular game are likely to want to purchase further entries in the series. This makes sequels a sound investment for publishers and developers who might not want to risk creating a brand new intellectual property when they know they can make a profit with another instalment.