Many fruits are quite sugary, and some people probably lived in areas where they could consume fruits regularly. Also, as another commenter stated, non-sugar carbohydrates can also pose risks.
The fruits we eat today are also highly selectively-bred to be very large and sweet. Take a look at a modern banana compared to a wild-type banana [1]. Same goes for watermelon [2].
[1] https://uploads-ssl.webflow.com/5e20f576d0e9d12be5abc763/5e3...
[2] https://i.insider.com/56ce1de88a45651707ff60c9?width=1190