The studios pay a lot of money for intellectual property (copyright) to make franchise films, that is why it make sense to use the opportunity to make more of these that pay off. For example, one of the biggest purchases in the history of franchise was Disney buying Marvel Entertainment for $4 billion dollars in 2004, thus obtaining the rights to use all Marvel characters created throughout the years of its existence.
Franchise films attract larger audiences as they are much easier to market: the potential audience is familiar with the characters and the story, so they are more likely to see the film. Therefore, it is not often so much about the quality of a storyline in every new movie but the attempt to use all opportunities that the familiar plot gives, and selling it to the biggest audience possible.
Speaking of the second part of the question - "movies get worse and worse" is quite a subjective statement. The film plots might indeed become less interesting but, for example, from the technical perspective films become better each time, thanks to the fast development of CGI industry.