https://www.livescience.com/32435-why-cant-humans-eat-grass.html

Lignin can be hard to break down by human stomachs. Not only that, but the lignin in food can actively inhibit the access nutritional benefits of other types of digestible fiber

Your dentist would not be pleased; grass contains a lot of silica, an abrasive which quickly wears down teeth. Grazing animals have teeth that are adapted to continually grow, replacing the worn tooth surfaces quickly

There’s usually just a handful of dangerously poisonous plants in any given area. If you learn those then most other stuff should be safe-ish.