We do not have 5 stomachs, so we cannot efficiently make use of nutrients wrapped in cellulose. Plants also protect themselves by producing things that are toxic (oxalates, lectins, tannins, sapponins, etc).

Sugar & carb consumption produces an emergency hormone response that produces fat. We do not burn fat well while we are producing it. The subsequent hormonal shift back to a more sustainable state produces a crash & hangry cravings. Cravings are compounded by the lack of bioavailable nutrition in plant products.

Eating fat (without sugar & carbs) forces your body to use fat for energy (& produce the small amount of necessary glucose through gluconeogenesis). This puts your body in a hormonal state conducive to burning fat, and makes the transition from burning dietary fat to stored fat almost seemless without a crash or hangry cravings.