How will I get my nutrition, my protein or be strong if I eat mainly fruit and vegetables?
Ever seen a horse or a gorilla?
Protein cannot be utilized by the body without being processed first. Processing generates waste that needs cleaning up and drains energy. The processing is needed to access the amino acids that the cells can use. That's why animals that eat meat tend to have a long rest period afterwards.
Fruit is the purest form of cell fuel as it is already in the prime state of amino acids, so no energy drain and cleaning comes as part of the package.