They are. At least for me.
I've got a couple thoughts on why that may be.
Hypothesis 1: Letting go.
I think this is the most probable explanation.
The gist is that if everything goes well, the good guys win, and they live happily ever after, there's actually still more to the story.
It's hard for me to leave it there.
If the protagonist just retires, that kind of seems like a sad ending to me.
Hypothesis 2: 2nd Law of Thermodynamics
Yup, we're bringing that into it.
In physics, it means that in a closed system the amount of disorder tends to increase over time.
It's why old barns fall apart unless you actively fix them.
It also means that, if your job as an author is just to play out a scenario on paper, without intervening on behalf of the characters, chaos wins.
Hypothesis 3: The Cheese Factor
This will be the hot new game show as soon as someone listens to my pitch.
Also, though, it's the idea that a rainbows and unicorns ending just doesn't seem real.
The guy gets the girl, they have a perfectly behaved kid, and never face a problem again.
I don't know, maybe the brightness settings on my soul are turned all the way down.
It's how I save battery.