There's a reason grass is so common - it's because it's a wildly effective life strategy. Grass is actually quite hard to eat - there's basically no nutrition in the leaves themselves, and grass evolved to incorporate silica "needles" in its leaves, so that it wears down your teeth when you try to eat it anyways.

Not to say that it's impossible to eat grass, but you need to undergo a ton of highly specialized adaptations to make it possible. For most animals (including humans), it's just not worth the effort