A while back, a fellow member of the Internet Writing Workshop e-list named Jeannette asked, “How does writing with obvious errors get through the editing/publishing process without being fixed?”
There are a lot of negative-nellies out there who will tell you it’s because no one edits books anymore. Every time I see or hear this statement, it makes me want to scream. Due to being short-staffed, it’s true many publishing house editors don't do as much (or enough) editing as they used to. It’s not universally true, though. I know many editors who still spend their train rides, evenings, weekends, and many afternoons at the office with the door shut editing their authors' books. Moreover, it depends on the kind of errors you’re talking about, because different editors (developmental, copyediting, proofreaders) are looking for different things at each stage of production.
There are many factors that contribute to a poorly edited published work.
1. A manuscript goes through several hands in a traditional publishing process including the acquiring editor, the author, the copyeditor, back to the author to resolve outstanding issues, the proofreader, sometimes back to the author again, the designer, production managers, etc. The process is designed to catch errors, of course, but sometimes all the futzing actually introduces them, especially when it comes to typesetting or any major revisions in the latter stages (many publishers charge authors for last-minute changes to discourage them from contributing to this situation).
2. Sometimes despite all arguments, an author may reject suggested edits. In these cases, the editor and publisher must ask themselves, "Is this edit worth cancelling the contract, upsetting the schedule, and retracting the catalog?" Oftentimes the answer is no, and so the editor allows the poorly written section to go through.
3. For various reasons (late delivery, production issues, desire of book chains or publisher, timeliness of project, etc.), a book's publication schedule may be "crashed," which means the publisher speeds everything up and rushes through the usual steps. As you can imagine, mistakes are made when you rush. Also, see reason #1, which is made even worse by rushing.
4. Some editors simply don't have time to edit their books as thoroughly as they'd like to, relying too much upon copyeditors to catch any lingering mistakes (some of which may not be within the limits of a copyeditor's job). And some copyeditors aren’t given enough time to do their preferred number of passes (i.e. the number of times they review the manuscript, often looking at different levels/types of errors), as described in reason #3 above.
5. I've heard a few times now from author friends that some smaller publishers have their editors doing double duty as copyeditors. This is probably done to save money, but it’s really a bad idea. Part of the purpose of having different editors doing different jobs is so that each can review the work with fresh eyes. The same editor doing two of the most important levels of editing defeats that purpose. Further, some smaller houses have gotten rid of their copyeditors and/or proofreaders as well as the accompanying parts of the production process, which means after that first look by the primary editor no one reviews the manuscript again until it's in print. Lots of room for errors to sneak through there.
6. Human error must always be considered. Publishers are human, too, despite what many writers think!
I don't mean to offer these as excuses: Certainly it's the publishers' job to produce books that are as clean, well-written, and attractive as possible. I expect clothing manufacturers to produce well-sewn and fitted clothing, likewise. That's what I'm paying for, after all. But maybe this gives you some insight into how it can happen.