The simple answer can be “Because it contains the substance chlorophyll, which is green”. But it does not have the necessary knowledge. The fact that chlorophyll absorbs everything except green is the definition of green. If you further want the explanation for this, then we may have to look at the better point of view, by considering all the aspects of science, which are Biology, Chemistry and Physics.
When studying the grass under a microscope you can see the chloroplasts containing chlorophyll which from the greek language is literally translated as a green leaf. This pigment has a green color and gives it to all plants. Under the influence of sunlight in plants photosynthesis takes place the main role in which it is carried out by chlorophyll. In this natural process carbon dioxide is converted to oxygen necessary for the existence of all living organisms on the planet. The substance absorbs all the colors except the green and therefore reflects only it. Thus the herb and many plants acquire this color.
Chemistry in its own way answers the question: why is the grass green scientist explaining any coloring of objects by the concentration of certain metals. For example the blood is red due to the content of iron in the hemoglobin. Vegetation has a green color due to the magnesium chlorophyll content. The most surprising is that with a plausible scientific explanation there is a mismatch. Replacing magnesium say zinc does not change the coloring of plants they still remains green.
From the physical point of view the coloring of objects is determined by the proportion of the received and absorbed colors to the reflected color. Everything that surrounds us has a color that reflects. For example if all colors are reflected then the color will turn white and if absorbed it will be black. This explains the fact that it’s better to wear light-colored clothes in hot weather and for example solar panels have a black color. Sunlight has a palette of seven primary colors that grass and other plants receive. It reflects only the green color which does not participate in the process of photosynthesis. Chlorophyll is much more profitable to extract energy for nutrition and growth from the violet and red spectrum and the green color gives away too much energy which can damage the pigment. This is how the exact sciences answer the question: why is the grass green now every reader can intelligibly explain to the child this phenomenon and memorize all possible scientific explanations of the habitual color of the grass under our feet.