They are made for people like me. Once I have dealt with the final boss, the game tells me what happened around the game world.
At some point, the game has to come to an end. It's an rpg with finite content.
Other open world games allow you to play after the main story is beaten, but at that point, if there's no meaningful new content, I don't care anymore. I move on to the next game. And usually, that content is more often than not side stuuf that is either procedurally generated, or some stuff that you may have missed.
I see Red Dead Redemption mentioned as an example. Other than being open world, I would say that the games don't have much in common. On top of that, one game had closer to 500 developers, if not more, working on it. Two or three times longer development period, and a budget that is not comparable. RDR is basically GTA in another suit. Playing them after story completion is fun for some people, but you don't change the world itself all too much.
Compare to a role playing game, where some things change the whole world on large scale levels. You would have to make content for that. As
JarlFrank says: You would have to basically make a sequel game for the game that you are already developing.
Are there better ways to end a game? Maybe, but they would probably be more expensive. Like allowing you to explore the world after the ending, you would have to add new dialog, reactions, environments and so forth.