It's because the majority of supermarket (and some lower quality greengrocer) fruit is picked unripe to give a long shelf life. Unripe fruit will often ripen after being picked - sometimes with the help of specific chemical gasses applied in the storage warehouses. Sometimes however they are just picked too unripe and they will never ripen properly. Instead of ripening to produce natural fruit sugars they produce more starch-like substances. This gives a wooly texture as you have found.