I initially reason, well, I must be a very unusual shopper. I buy a lot of books on programming topics, math, science, etc., and I eschew with passion anything written for dummies, idiots, fools, or morons.
But then I wonder, am I really so unusual? I'm not an expert historian, but I enjoy studying history, and I'm disappointed with the local store's selection of history books about as much as I am with their technology books. I enjoy studying linguistics, and have watched the store's linguistics section melt away into nothingness. I enjoy science fiction, but am dismayed to see shelves of fantasy displace books by my favorite authors.
I can't really speak for every subject matter, but for pretty much every topic I want to read about, the big chain bookstores seem to be dwindling their stock, and smoothing out what little they do carry to appeal to some unknown typical readership.
If this is indeed true of all (or even most) of the subjects of books they carry, then I find it hard to believe that anyone who truly enjoys reading books is content with what they carry. Which leaves... people who really aren't very fond of books to peruse the shelves? I don't know. It doesn't make any sense to me.
I would greatly enjoy doing more book shopping in person. I seem to be good enough at searching Amazon to come up with good books sight unseen, but I'd like to go browse in the store, if only the store had books in stock that I'd want to browse.