Firstly, because they chain kids into reading books by their reading level, not by interest or appropriateness.
Secondly, because they MAKE NO SENSE.
So, a kid about thirteen comes in. He is supposed to be reading around 1000 level. He's "allowed" (I gnash my teeth) to read down in the 900s. He loved the Hobbit (L1000). Can he read Fellowship of the Ring? Nooo, it's L960. But he can read Paul Hutchens Sugar Creek Gang series! L1500.
And why is this truly ancient series L1150? Because Paul Hutchens wrote fairly short stories about Christian boys and their adventures without any punctuation and his sentences would go on and on for sometimes more than a paragraph without a break and this apparently makes him a more challenging author than Tolkien.
So, we return to the book of lists. What about something called Ghost Liner? It's L1100. It's also only 64 pages.
"What's this, The Native?" Trust me, you wouldn't like Thomas Hardy.
How about Gordan Korman, I like him. Nope, too low.
I did finally send him off with T. H. White's Once and Future King and Steve Sheinkin's Two Miserable Presidents, but....
It is a wonder any children still retain the faintest interest in reading after this. E. B. White wrote a hilarious and pointed essay (the title of which I cannot remember) about checking his writing on a fancy new machine that measured the difficulty of your writing. The machine's conclusion? White's writing was practically infantile. The machine's instruction booklet was measured too hard for anybody to read.
Oh, and the whole "it challenges kids" argument parents keep giving me? Toss your reading levels and bring your child to me. I personally promise to challenge their reading ability AND find them books they actually want to read.
Oh, and btw, I actually rather liked Sugar Creek Gang when I was a kid and the updated editions aren't bad. But still in no way comparable to LOTR