If I remember correctly, NA was one of Austen’s earlier novels, so it could in part be due to her still growing into her style and her craft, which probably includes knowing how to satisfactorily wrap everything up.

There’s also the possibility (though this is 100% a guess and I can’t say for sure) that the types of novels NA was both emulating and poking some fun at (the sort of exaggerated gothic romance type novels Catherine was so fond of reading) have equally abrupt “and they lived happily ever after” endings, so it was all part of her matching that style? But like I said, that’s 100% a guess on my part.