Only they're not.
Lots of sequels improve on their originals.
Here are some good movies with even better sequels.
Frankenstein and Bride of Frankenstein.
A Fistful of Dollars and For a Few Dollars More.
The Godfather and Godfather II.
Night of the Living Dead and Dawn of the Dead.
Toy Story and Toy Story II
The problem isn't with sequels. The problem comes when the series gets extended into a trilogy.
The Good, The Bad and the Ugly just about holds its own, but Godfather III and Army of Darkness show a distinct falling away of inspiration. I forget what the third Frankenstein movie was called, but I know I've seen it and it wasn't much good.
The original movie is experimental. The sequel gets it right. The third is almost always a mistake.