We need even more snuggles
Most other manga: The story must end once the main couple gets together because the shift in their relationship is hard to write.
Alternatively it's the end of their intended narrative, and the story they want to tell. How a couple got together, where the story was the journey to that point. Which can be fine in my opinion, though I definitely agree it's often nice to see past that point, and their life together as a couple.