A big theme in education is that short of very basic knowledge you want to teach from nontraditional educational resources.  Exceptional teachers will teach with materials they have either created themselves or pulling from the community. Using "the book" only as a last resort.  You want to pull in outside resources and then relate it to the content.  From what I've seen in educational games whether online or standalone programs they tend to focus on the very basic content knowledge and are essentially flashcards with a really shiny, and expensive, overlay.

Time is also your enemy when it comes to educational games.  Most teachers use it as an additional resource and needs to be short.  So using video games most effectively is already being attacked at.  Also bringing any sort of media and presenting it as educational is very hard outside of books.  My district has put a soft ban on movies or tv even. Not even bill nye :(.

Educational materials are a big big big big big big big business and districts, along with states, are all looking for ways to fix their problems through everything but hiring enough teachers..  If you offer a cheaply made program with high visibility you will make money.  The motivation to create great games is just not their.

(I'm starting to feel rambly so I apologize Taeja vs Innovation is distracting me.  TL:DR:  Schools are not set up to use video games effectively as a teaching tool because they are pain to make sure students have access to and can explore as in depth as they need, you can't make a reference if they don't know it.  Video games are innately very rigid when it comes to experimenting with them, requiring heavy knowledge in moding or creating their own games.  Their is no insentive for educational game developers to make anything more in depth than fancy flashcards.  The money is too good to ignore and their is no call from the major states to try and use it any other way. ANDDDD bloom's taxonomy is boss mode!)