They do this because it works and makes money. They're just giving us what we want. Take a look at the top grossing films of 2017
http://www.imdb.com/list/ls066822614/
Of the top 25 only the following were not either a sequel, part of a series, or based on already existing intellecutal property:
#15: Dunkirk
#20: Coco
#24: Life
#25: The Great Wall
That's it.