it's about money. if you can put in minimum effort and still attract a pretty broad audience, why the fuck not? people are going to pay to see american pie 5, transformers 3, halloween 8, the fast and the furious 6. it's probably dead fucking easy for them to write a story based off previous movies, so costs are probably relatively low, and they still pull in a fuck ton of money.
i think it's stupid too, but that's the way the world works. it's the same reason one company comes up with a fucking new idea for a phone or ipad, and every other company has to rip it off and make a nearly identical version with like 1 different feature and a different name. it's easy and cost effective to branch off something that's already out there. is it shitty? yes. is it less creative? yes. is the world just one big cash sink? yes. is it going to change? no. do i still benefit from the entertainment it has provided me? yes.
just don't worry about this kind of shit. the world is driven by money, and that isn't changing.