Nature has created each species with a unique print.

The makeup of our blood  is alkaline and hence, in our diet, we need vegetables also which are alkaline in nature.

It is out of our years of observation that persons who took to fruitarian diet for over three years became very weak and had to revert to the habit of adding more vegetables in their daily intake so that they could gain back their lost strength.

One may argue that monkeys are fruitarians (for the most part)  and are very active, And since  their genetic makeup is close to ours, we can also be fruitarians.

Our lifespan is different from that of monkeys. Even the very large monkeys do not live beyond 35 years (approx). Most monkeys  live only for 20 years.

Humans can also be very active in all dimensions if we put our attention in that direction.

Also veggies have more micro nutrients than fruits and also are less energy dense where as fruits are more on glucose and fructose and a bit more higher on energy.