because you can bet the original
They don't always.
"A Shot in the Dark" is widely seen to be better than "The Pink Panther"
Empire Strikes Back is seen by some as better than Star Wars.
"The Good the Bad and the Ugly was the best of the Dollar trilogy.
Aliens which you had mentioned.
Sequals would seem better if you had not seen the original, because so much of a movie series is formula, the formula is better when it seems new.
because sequels are just trying to milk the success of the first movie (unless it's a planned sequel which most aren't) and therefore plot/story/everything else that makes a movie good doesn't matter as much as it did in the first
Godfather Part Two was awesome, however not as good as the orig
if the original was good, they dont have to be good. they will make alot of money just because people will go see it no matter what. Look how much money starwars episode 1 made and that movie sucked.
maybe because your in a time of not so great sequels.
Star Wars had great sequels
God father
and many others.