Because banana peel is not very good packaging, or compared to man made. Good enough for its own purpose. I've vacume sealed bananas with plastic and they stay fresh for maybe even a year, that's the difference.
I watched a documentary recently that compared frozen food to internationally shipped food and found significant nutritional loss in the shipped "fresh" food. As such, I think plastic preserved apparently fresh food could be an explanation for part of the obesity rise. One potentially consumes more calories to get the other nutritional factors that have broken down compared to eating food that can only be delivered actually fresh.
(Given that improved shipping life also turns the market for a vegetable global, I also think more people are priced out of buying produce that once had to be sold locally than are helped by less produce waste in the richest market the produce can now reach.)