The reason for this is that our metabolism was "designed" for our Paleolithic ancestors, for whom every day was a contest to try to find enough food to survive. To find a source of sugar--an immense source of calories, which under normal circumstances were not easy to find--would have been cause for celebration!
But the Neolithic Revolution--the discovery of the technology of agriculture, which happened only a mere 12,500 years ago--began to change this. Our distant ancestors learned how to domesticate animals for their meat, and also how to cultivate certain plants that were very generous sources of nutrition. Berries and other plants whose fruits are high in fructose were, of course, the Stone Age equivalent of gold!
It wasn't until the Iron Age, which dawned roughly 1000BCE, that our ancestors were able to construct tools that allowed them to cultivate plants which were too difficult to work with, using the older, less sturdy and less precise tools made of bronze.
