The early themes of these shows tend to be about comedic struggles, absurd contradictions, and fumbling romance. You can do that for a while, but eventually you have to show some kind of character growth. Otherwise you run out of material. Grow the characters long enough and you run out of adversities to throw at them.

British shows get around this by doing, like, 3 to 6 episode seasons that last a few years. And even then they run into trouble later on. American TV does 13-26 episodes and run on for half a decade or more. Just compare the British and American Office. Or British and American House of Cards. Happily Ever After is just how you terminate the plot. "Look! No more jokes! Everything is resolved now!"