Same reason there are a 100 Call of Duty game sequels. Low risk. These projects involve millions of dollars. The people in charge could risk that money on a really promising story from a nobody. The risk could be worth it and everyone involved makes enough money to build their own Scrooge McDuck money vaults. The project could possibly fail and that failure would blow back not just on the decision makers, but on the entire company. We all know of gaming companies and movie studios who went bankrupt from just one really bad movie/game. A sequel, on the other hand, might not make mind-boggling amounts of money, but it will make a predictable amount of money. Low risk and everyone gets paid.