From evolution.

Plants are living organisms, and they do not want to be eaten, so they have evolved many defences to that end. They cannot run away nor physically fight back, yet they are one of the most successful kingdoms on Earth.

How do plants protect themselves? Their primary form of defence is chemical warfare. Plants produce chemicals like oxalates, lectins, phytates, cyanide, hormone disruptors, nutrient blockers, and carcinogens to discourage animals from eating them.

Animals and plants have been evolving together in a never-ending evolutionary arms race for millions of years, wherein animals develop adaptations to be able to break down the plants' defence chemicals safely, and plants evolve stronger defence chemicals. In nature, we see this manifest in herbivores being very specialised in the types of plants they can eat without getting sick. This is why we don't see every animal desolating entire swaths of forests, marshes, grasslands, etc.

Humans, too, are animals, and it was only in the last 12,000 years or so when we invented agriculture and settled down, thus entering a new age of heavy plant intake. Almost immediately, we experienced negative effects such as a shrinkage of brain size, a shorter stature, and poor teeth health. However, while relying on plants at the individual level resulted in health sacrifices, especially later on in life, at the societal level, agriculture provided a means to dramatically increase a settlement's population size and strength.

Humans still instinctively know to not eat plants unless necessary to survive. For example, if you were thrown into the middle of a forest, you would know that eating most of the plants around you will immediately make you sick. Parents also frequently see this when they force their kids to eat so-called healthy foods such as broccoli, spinach, and Brussels sprouts, which the kids will intuitively avoid, but are forced to accept in the name of health.

Essentially, each species has a species-appropriate diet, and humans are not special. We have specific adaptations for specific foods for optimum health, just like every other species — we've just forgotten what that is.