That’s where the seeds are. Like a cantaloupe or pumpkin. Fruit is the seeds surrounded by fleshy pulp of some kind. They all grow in a big cluster, so them being narrow and long can fit more in the same area.
https://cdn.britannica.com/80/98480-050-EF237E5F/Clusters-papayas-farm-Laie-Hawaii-percentages-production.jpg
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