Irrational games is a very special case: the "distance" between 4.0.0.2 and 10.1.0.0 is actually not that big, and they have used in both games significantly not modified (original) game engine (I guess they are simply using nif converter that is included in the package). On the other hand MW (4.0.0.2) and OB (20.0.0.5) have very different nif formats and both games have several serious modifications and additions (e.g. FaceGen implementation in OB, released by NDL viewers and Max exporters are not totally compatible with MW nifs). Such conversions will be simply not professional: MW models in OB will look too out of place primitive, and OB models simplified and too heavy for MW performance.
We are in the "bad" situation: NDL/EGT owns the engine but produces no games; Bethesda (and others) produce games, artistic resources but have no full rights for the engine, programming. Another situation is with e.g. Quake, Unreal, Crytek engines and games, these guys have available more advanced modding tools, regularly updated exporters. Moreover, codes of old versions can be released (complete codes for old Quake are available for not commercial use).
Anyhow, they have rights and can do whatever they want. They are serious informed interested people and have sensible reasons to do or not to do certain things.