Are Children Smarter than Preteens?

YA fiction annoys me. It's hard to explain why, but I'll try. Whenever I read it, I get so irritated; I feel like the author is condescending me. The sentences are so short and basic, and the authors don't seem to know how to show rather than tell.
I'm sure not all YA is like this. I've only read a few. The ones I have read usually have good plotlines and interesting characters, but the writing style is always the same, regardless of the writer. I don't know why this is the case.
Children's fiction doesn't seem to be like this. Books like The Hobbit and Harry Potter don't suffer from this "talking down" to the reader problem. For some reason, society seems to think that small children can comprehend good writing, but preteens must be given this awkward easy-to-read stuff. As if we get stupider as we get older.
Have you ever noticed this (if you've ever read anything labelled "YA")? Do you think this is good or bad?