Until you've edited a long RPG book, you don't know what a mess it is of multiple considerations and kinds of consistency, changes with domino effects, conflicting priorities. Your editorial faculty eventually gets numbed to the content and the phrasing. It just takes a lot of skill and a lot of time, and in the RPG industry -- undercapitalized and understaffed (including at Wizards) -- companies decide to release books with tolerable levels of errors rather than spending more of their limited funds to remove, say, half of those last mistakes, which aren't perceived to seriously affect sales.