Here's some possibilities (why RPG editing sucks):
1) It doesn't matter enough... these books aren't rocket science, nobody will die if there's an error in it. Most of them are still useable to some degree.
Just to get totally off topic, rocket science books are horrible for editing. Usually the editor ISN'T a rocket scientist, and the equations are botched. Naturally, the equations are the most important part.
The most precious books in my professional collection are the hand-me-downs, with all the errors lovingly corrected by previous owners.
The d20 Modern book is pretty well done, in my opinion. I've read about 90% of it, some of that multiple times, and while I've noticed a few mistakes, I can't recall anything major. I'd consider it a fairly safe purchase (and a worthwhile one too).
In general, I agree that there's a lot of bad editing in this field. One thing that I used to notice a lot (especially when I used to read TSR novels) that editors seem to miss is the distinction between east and west. No one over messes up north and south but for some reason it seems like if you compare text to maps, about half of the east/west references were reversed. I haven't noticed this as much lately, but then I don't read nearly as much campaign world related stuff these days.
