Just because there are grasses ( Poaceae ) with edible and nutritious parts does not mean that this applies to all grasses.
That is pretty common-place.
A quick Google search give you all the info you need: Many grasses are edible, in the sense that you can eat it; you simply won't get any energy/nutrients out of it.
They consist mostly of cellulose, and our digestive tract is simply not made to break that down (as opposed to, e.g. cows, which employ four stomachs for that purpose).
There are however also grasses that are toxic.
So when not discerning between actual species, the effect of eating grasses is somewhere between no effect and harmful.
outdoors.stackexchange.com/users/3602/imsodin
Eating grass isn't smart because our bodies lack the enzymes to digest it, and because you never know if some animal came along and crapped on it.
Don't eat grass.
outdoors.stackexchange.com/users/14336/Caleb Way
To add to the existing answers, depending on where the grass is growing, there can be all sorts of synthetic fertilizers thrown onto it which would be very harmful to humans.
You do not want to eat Scott's Lawn Fertilizer.
outdoors.stackexchange.com/users/14343/ClydeTheGhost
Apart from the low nutrients issue, grass stems are covered in tiny silica spikes, which act as an abrasive on your insides.
These spikes are thought to have evolved as a defense against being eaten ( Silica in grasses as a defence against insect herbivores: ) -- which clearly didn't work in the long run.
But anyway, if you don't have a tough lining like a cow has on its lips, esophagus and stomach, you'll risk feeling sandpapered inside or worse.
outdoors.stackexchange.com/users/13206/Elise van Looij
A look through the veterinary literature can show you some further reasons not to just go out and eat grass.
Specifically, parasites like liver fluke, different species of tapeworm, and some nematodes can infect humans as well as sheep (or cows, or mufflons, or whatever lives on the pastures where you find your grass).
So, in a real outdoors situation, experimenting with grasses is unwise, even if you are very hungry.
outdoors.stackexchange.com/users/506/rumtscho
A recent comment    Cows eat grass, doesn't mean you can!   got me thinking.  Corn, wheat, rice, etc are all grass that we eat regularly.  I am not sure the human race could survive on the planet without eating grass.    But in all fairness, for the most part humans only eat the grains of the grass.  Can I eat the other parts of the grass also?   If yes, what do I need to know and/or do to prepare the grass for eating?
Can I safely eat grass?
outdoors.stackexchange.com
<safety><food><plants><edible>
outdoors.stackexchange.com/questions/17555
outdoors.stackexchange.com/users/4079/James Jenkins
Experimenting with grasses is unwise, even if you are very hungry. Even though there are grasses with edible and nutritious parts you simply won't get any energy/nutrients out of them. Apart from the low nutrients issue, grass stems are covered in tiny silica spikes, which act as an abrasive on your insides. We  don't have a tough lining like a cow has on its lips, esophagus and stomach. Our bodies lack the enzymes to digest it, it may be contaminated with fertilizers or animal waste. Parasites like liver fluke, different species of tapeworm, and  some nematodes can infect humans as well as sheep or cows. Do not eat grass, it can contain fertiliser, parasites and can be toxic. Humans do not digest grasses well and they have no nutritional value.
Don't eat grass, it can contain fertiliser. Grasses do not have nutritional content. Grasses can be toxic, contain parasites and humans do not digest them well.