I can go along with you on that one (I know Dana Carvey was really wanting a sequel to The Master of Disguise), and while that actually can be the case some times, it seems more like in this day and age, sequels for movies has pretty much become an overused and overrated gimmick... y'know, like 3D? Also seems like the fact that some movies already have sequels in the works right when the first just comes out kind of speaks volumes, but then again, like you say, some of it may have to do with hooks and clauses and such.
Still, it almost seems like it's gotten out of hand.
I don't mind as long as we get a story that needs a sequel. There are films that have better stories in the second film, especially since they got all the establishment stuff out of the way. Kung Fu Panda 2 was a far deeper and darker movie than the first. The villain of the first one was just someone that turned evil and wanted revenge. The second one was about a peacock from a crappy family that went nuts and wanted to cause mass panda genocide, all the while diving deeper into Po's past and why he was an orphan. The Toy Story films got deeper with each sequel. And why there are those that disagree, I think Back to the Future 2 was overall more enjoyable than the first, and completely original being the only movie at the time to go back to the first one and create a second story arc that co-exists with the first film's. The first one was about choices, sure, but a lot of the film was reverse Oedipus Rex style, while the second dealt with consequences of screwing around with time. Not only in the past, but the future.
Then you get those sequels that are a waste of perfectly good potential. Cars 2 was clearly meant for children, but they could have had a more meaningful plot than "super spy movie parody#5624." As much as I liked the oil conspiracy theory in it. Chipmunks 3 was just a "WHY?" in terms of the plot.
And my personal favorite, Hangover sequels. Hangover 2 was a tongue in cheek joke about how it would be impossible to make a sequel to certain films because they'll just wind up making the first film again with a bigger budget. The film was basically saying "how can anyone be stupid enough to let the same events happen twice?" And I don't think anyone even got that joke, cuz they're making a second one.
But we all want to see more story. Problem is, we wind up envisioning better than the film writers do a great deal of the time.