It is said after the credits end that you can continue to play the game but the world is in the same situation it was before the last chapter, and you are denied access to the main quest. So, yeah wouldn't make sense.^^
My guess as to why this happened is that the devs probably felt pushed about allowing gamers to play after the end of the story while they had first stated (IIRC) quite some time ago that it would bring the game to an end.
In my opinion I'd have much prefered the game to 'really' end, not allowing you to continue playing, which would have given a better feel of closure. But no choice would make everyone happy and I understand that! Because I value the story so much that I put it above the game itself (if that makes sense), I would have prefered a "true ending", but I know that for some others, being unable to continue playing after the end of the story in an open world game would feel really strange.
Just my two cents!