Thread:ADour/@comment-5977347-20130426012007/@comment-1713281-20130426014812

I don't know about handbooks (though it wouldn't suprise me if that were the case), but the general answer is that they are kept in the same universe, well, "because".

Video Games tie-ins are made for the purposes of being in the same universe as their source material, and as such we leave them in the same universe as the source material. Exceptions have been made (very rarely), but it just creates massive issues if you go around making a new universe for all the video game tie-ins (by my count, that'd be almost 15 new universes).