Because we don't have any enzymes like lipase or proteas to break down the grass molecule so it can be digested
Cellulose (plant fiber)
Humans lack the necessary enzymes to efficiently break down cellulose, the main component of plant cell walls, into digestible sugars. Our digestive system is not equipped to process cellulose as effectively as herbivores like cows or termites, which have specialized gut bacteria to help digest cellulose.
Without cellulose in its cells, a plant would not have the necessary structural support to maintain its shape and grow upright. Cellulose also helps with water and nutrient transport within the plant. Without cellulose, the plant would likely wilt and eventually collapse.
plants use cellulose as a way to keep the stem sturdy
Horses break down cellulose by use of enzyme cellulase.
In cows and other herbivores, their natural flora of bacteria break down cellulose and make it something they can digest, but in humans our bacteria have no effect on cellulose, so for us it is indigestible.
The only weakness these creatures had was their inability to digest cellulose.
Yes, the breaking down of cellulose by monerans can be beneficial because it helps in the decomposition of plant material, recycling nutrients back into the ecosystem. This process aids in nutrient cycling and helps to maintain the balance of ecosystems.
It is a good source of fiber.
Feces is a product of the inability to digest certain types of food. Due to the diversity of organic nutrients and structure, it is impossible for an organism to break apart every type of nutrient encountered. One example is cellulose ("fiber"), which is a carbohydrate. Cellulose is found in the cell walls of vegetables. A human digestive system cannot hydrolyze cellulose (certain prokaryotes can), and the cellulose is passed through the alimentary canal as waste/feces.
Plants use glucose monomers to make cellulose, which is a structural polysaccharide found in their cell walls. Glucose molecules are linked together in chains to form cellulose polymers through dehydration synthesis reactions.
It varies from year to year, of course, but they use around 5 million nutrient tons a year on average.