Series that suck after the first few books

Why would the later books in a series be considered bad if they were written by the same author who wrote the initially good books of that series?  It's the author's story as the author intended.  Surely, the extra books add to the narrative as a whole.  I cannot see why later books in a series are considered something to ignore, when the earlier books were considered excellent.

Jack Whyte's Camulod Chronicles, Robert Jordan's Wheel of Time, Orson Scott Card's Ender series, Terry Goodkind's Sword of Truth, Tom Clancy's Jack Ryan novels, Laurell K. Hamilton's Anita Blake, Anne Rice's Vampire Chronicles, Christopher Paolini's Inheritance, and even series with rabid fandom like Left Behind or Twilight, tend to regard the first book as excellent, up to about the third book of the series, and then notice a tremendous drop in quality.  Why?

I disagree. I think the Wheel of Time series got better. It was not until the fourth book that shit actually started happening. The first three books were really just giving us an idea of what the characters were like and what world they lived in. 4-7 was when shit was getting ready to go down. People were priming the grounds, Rand got his fat ass armies, whats her name died, the bodyguard fell in love with that girl and helped her kill all those niggers back in the hometown, and the magic girl became head of the outlawed blue and green wizards. then books 8-11... fucking shit gets thrown down, the seanchan straight up invade from their continent, matt falls in love with the queen of the seanchan and works his magic on her, rand goes super saiiyen level 9. Feels good