I don't think the licensing is really the excuse. It is more of the fact that studios have put tons and tons of effort towards those assets including their creativity towards that partly finished end-goal. They most likely want to keep it in their possession if some point tides turn and they are able to finish the game or even small part of it.

And I totally understand that. After sacrificing years and years + thousands/tens of thousands euros towards project, you really don't want to see it taken by other people and putting their own creative ideas on top of your work(that can be potentially used later).