The ape/human digestive system is evolved for a vegetarian diet. Meat was added later and the addition of protein to the diet probably added to the eventual increase in brain size.
Chimpanzees eat plants, insects and meat. They also hunt in groups using clubs.
Also in most places fruit is only available for a few weeks or months at most each year. Survival on a diet of fruit is not practical. Even in the tropics there just isn't enough fruit for it to be a primary source of nutrition.
Eating animal flesh and fats provides the long chain fatty acids (DHA & DTA) required for brain growth. Additionally a nutritionally dense diet requires a less energy-intensive gut which frees up energy for the larger brain.
The four most expensive (energy-wise) organs are the heart, liver, gastrointestinal tract, and the brain.
The organ-body mass ratio of heart and liver are roughly similar amongst all the great apes, the main difference is the GI and brain ratios.
Meat eating humans have the smallest GI tract and largest brain. The fruit (& meat) eating chimpanzee has a smaller gut and larger brain than the leaf eating gorilla.