Because of it's history.

You see long ago, when humanity was young, there were the Plant Bloods. They were humans corrupted by the greatest plant in existence, The Blood Mother. This monstrous being was as much animal as plant. Unlike other plants, it could think and feel.

They called it The Blood Mother because it had bright red blood flowing through it, which gave it's green skin tinges of red at it's thinnest points. It's roots tentacled throughout the entire world. And from these roots it grew small fruit bearing trees.

When an unsuspecting human went to eat this fruit, they were infected with the parasite. This parasite slowly turned them from human, into a human plant hybrid. Their hair would turn to vines or flowers. Their teeth to thorns. There minds would be connected with the Blood Mothers.

These plant bloods could only get so much nutrients from the sun. So they turned to more animalistic means. More cannibalistic. They would use pheromones to lure normal humans from their camps, and feast on the poor souls.

This continued for centuries. But one fateful day, a woman was widowed when a group of Plant Bloods devoured her husband and daughter. She swore revenge.

She traveled the world to learn what made the Plant Bloods. It was her that discovered parasite and the Blood Mother's trees and roots. She spent several decades tracking the roots back to their source.

She made a sacrifice that would save humanity from the Blood Mothers parasitic ways. The woman burned the ancient plant to cinders, but could not escape the flames herself. She had given her life for revenge for her family, and to free the world of the Blood Mother.