In a post about agents and editors, Peter L. Winkler asks, "[W]hy is is that with so many 'dedicated, professional' gatekeepers, so many bad books get published?"
I love this question. It's not that I have the answer, because I don't. Nobody does. It's because there are so many different answers, and they're all correct part of the time.
Bad books get published because...
1. There's no arguing with taste. Your dreck is another man's page-turner.
2. A comedian once said, "If you were talking with Einstein, and an alien ray zapped him from the sky and suddenly made him twice as smart, would you be able to tell?" He went on to talk about how you don't need to be incredibly smart to come off as smart. You just need to be a little smarter than the guy you're talking to. In the same way, a book doesn't have to be brilliant to come off as good. It just has to be a little better than the reader's ability to tell good books from bad. Most readers have less developed literary sensibilities than the average critic or writer, so a larger body of books look perfectly good to them. Editors are willing to publish these mediocre books because...
3. A little bit of badness is a good thing. Think about the writing style of the average pop hit. Crap, yes? Simple, full of cliches. Tinkertoys writing. But people love these books because they're easy to read--people don't want to work at their entertainment.
In some genres, badness is also good because it encourages active fandoms. Readers love books with plot holes and shoddy characterization because it makes the book more of a blank slate for them to draw their own ideas on. When they feel they can play in the book's world freely, they get together to swap ideas, play together, etc., and *poof*--instant fandom, a free grassroots promotional engine. Nowadays this is more true of TV and movies than of books, but occasionally a Harry Potter or Eragon comes along.
4. Publishing slots are time-sensitive. If an editor needs six slots filled by the end of April and she has only five good manuscripts, she'll grab the best of the rejects because it's better to put out crap than to leave an empty slot. The problem's inevitable in magazine publishing, but I don't know why book publishing works that way. The likely answer is that production and distribution departments are set up to handle a certain number of slots per month, and leaving a slot empty wastes money.
5. You never know what will sell. Every so often a piece of garbage hits it big, and even the less explosive books often turn a small profit. May as well use that empty slot to take a gamble.
#2 and 3 could be improved, though not fixed, with extensive education. If we're going to strap the masses into desk chairs and shove some learning down their throats, though, literature shouldn't be high on our list of priorities. Food for the soul, highest effusions of the human heart, yeah yeah whatever--can we teach them how to find Canada on a map first, please? And maybe give them some basic medical knowledge? And phonics? As much as I love books, they're not that important compared to the gaping holes in most people's educations.
Besides, a love of trash is human. Ever since printing presses started mass-producing books we've been swamped with garbage. And why not? Garbage is an easy read, and sensationalistic writing is fun. Do you really want to be stuck in a world where the only reading material is Proust and Joyce? Even if we reformed literature until every week brought a new flood of brilliant, subtle writing about the deepest reaches of the human heart, we'd still have a solid bedrock of vampire romances and space operas.
In the end, the reason we see a lot of bad books is because people read them. Always have, always will. If the big publishers stop printing lowest-common-denominator books, small publishers will slide in and fill that niche--which, let's admit it, is the niche where the big money is. On top of that, there's no accounting for taste, and the gatekeepers know it. Plenty of books that have been rejected as garbage have turned out to be bestsellers, so who's to say that this latest piece of filler won't?
Besides, if you had to read slush, you'd start thinking crap was gold, too. It's all in the comparison.