Sequels or spin-offs are not cheaper to make. The allure in them comes from the fact that an established series or franchise will perform relatively the same throughout the multiple reiterations and reincarnations it undergoes because producers and investors would rather put their money towards something they know will be a decent success and have some net profit. So they stick with the same formula most of the time in order to recover their investments. Sequels are often much more expensive to make.
And there are plenty of independent films that buck this trend, yes. But don't count out the other mainstream films, i.e. Inception, Kickass, etc, that are unique. Out of the hundreds of movies made, only a few can be original and that's normal. Nobody is forcing you to see Shrek 6 really.
Last point is that without reliable franchises to make money off of, there wouldn't be funds for the actual good movies we all rave about. For instance, without the success of Batman Begins/Dark Knight, Nolan would not have had the clout nor money to make Inception. While this might be a bad example since the Batman reboot was very unique in its production, it still conveys the system and idea.
