Hollywood, being the unimaginative people they are (-cough- yes, I'm biased), just want the fast buck and that comes from sequels. And most of the time the sequels are a waste of money. Someone mentioned that the LotR sequels aren't counted, and I'll have to agree because they were filmed together. Other than that, the only sequel that I've watched (I heard Spider-Man 2 was better, but I didn't watch it because I hated the first one) that's on par with the original - not even talking about being better - is Before Sunset.
Is there any wonder that movie attendances are falling?