Listening again I’ve noticed how the piece uses orchestration to tell the story.
The melody is carried by sounds from the 40s – muted trumpet and solo violin. Later we hear vibes, probably suggesting domestic life in the 50s. Notice how the pizzicato strings are used in a way that sounds like the 50s.
What we never hear is a sweeping section of violins carrying the melody. This is no John Barry score from Out of Africa. That’s because Carl and Ellie never got to have their travel adventures. Flourishes of strings are limited to transitional effects and counter melodies.
This restraint keeps the cue focused on the relationship of Carl and Ellie and sets up Carl’s motivations to fly to South America.
By the time we hear the solo piano at the end (and everyone is bawling), we’ve heard the hook 8 or 12 times, showing there is nothing wrong with a lot of repetition.