The same reason people want a Jurassic Park IV and Indiana Jones V, I suppose.
Oh
bad examples dude! The two things you mentioned, lend themselves happily to further stories. There's room in the universe for them. In the case of our favourite extra-terrestrial, the story was told.
Bish, bash, bosh DONE. Simple as. No further exploration, explanation, extrapolation needed. DONE. OVER.
I want to be a writer. I want to write for young audiences. And I write what I call "BIG STUFF". Not "sci-fi", not "fantasy" but "BIG STUFF". If I am a success, I won't be like J.K Rowling and write 7 or 8 books about the same thing. I will do NO SEQUELS. EVERYTHING I write will be original and stand alone.
Whilst I like a lot of sequels, it isn't what I want to do. I'd NEVER write one, and it's Spielberg's attitude to his classic movie that has inspired me to be like this.
The world needs less sequels. Simple as.