I dont understand what makes you think a natural fruit without any human intervention couldnt be sweet. they already sweet, but our research can make them beyond just being sweet. easy to eat, bigger, stronger, and even making them limp and preventing the seed to be replanted.
I guess your standard of sweet already warped by all the chemical stuff like glucose syrup or fructose. thats why you cant imagine an overly sweet fruit that doesnt leave bad aftertaste in mouth. I hope you can try theese kind of fruit in life since they are actually amazing. my suggestion, try mango.