I can't find a source quickly -- but I remember reading that many years back (like when we were apes in Africa), the sweetest fruit was probably as sweet as a carrot. We increased the sweetness of fruits through selective breeding to current levels.
A quick browsing indicates that while our modern fruits are all domesticated, there are plenty of wild, sweet, and large African fruits. Apparently some modern wild berries also have similar sweetness profiles as their domesticated counterparts.
The article points out that one prominent difference that does seem to exist is that wild fruits are harder to eat, due to big seeds, thick and tough exteriors, and lower water content. So you might have to do a lot more work to get the good stuff with wild fruits.
I've eaten wild Muscadine grapes in Georgia that someone found in the woods (I'm not sure if this counts as a wild fruit or not due to potential Native American cultivation, but close enough imo), and they were fairly sweet. They had thick skins and giant seeds in them, so they weren't great actually, but they were sweet.
That doesn't explain blackberries which have only been cultivated in the last 100 years, due to more advanced farming techniques. They have been seen as more of a scourge for centuries, because of their knack for strangling other cultivated plants.
And I imagine the fruits wouldn't be eaten as ripe as they are now - people prefer sweeter fruits, and they sweeten as they ripen. I prefer more sour and tart fruit, so I tend to prefer less ripened fruit.
