I've often been curious about why humans don't simply eat grass. We're surrounded by the stuff, yet people starve to death anyway. Why don't rich people employ the homeless as lawnmowers?
I found my answer here
Why cant humans eat grass?
Cows can eat grass, so why can't we? Well, plant cells contain something called cellulose. Chemically, it's almost identical to a lot of the carbohydrate molcules (like glucose and fructose) in the food we eat. But cellulose has one bond that's different, and we can't break that bond to get the energy out of the cellulose the way we get it from other molecules. So we could eat grass, but we wouldn't be able to digest it to get the energy we need. Cows and other mammals can eat grass because they have special bacteria living in their stomachs called symbiont bacteria. These symbionts can break the bond in cellulose for the animal its living in. That way cows can get energy from grass. We don't have any of these symbiont bacteria, so we can't digest the cellulose in grass.
The more you know.