Because that's just what it is. A plant. The name grass is just a better way of explaining a type of plant that is tough, durable, creates a healthy sod for the earth, and is a favourite plant for grazers like cattle and sheep to live off of.
Corn is actually a grass and is a monocotyledonous plant. The corn kernals are the seeds of the plant.
Spinach is not an herb or a shrub. It is a leafy green vegetable that belongs to the amaranth family.
Wheat (the plant) is, as Cycling-rod says, a cereal grain, which is a type of grass grown for its "fruit". It's definitely not a shrub (a small to medium sized woody plant). It's also not an herb in the culinary sense (a plant grown for its aromatic or savory leaves or flowers), though it is a "herbaceous plant" in the botanical sense (a plant with no persistent woody stem above ground).
Gulmohar is neither a herb nor a shrub, it is actually a type of tree. Its botanical name is Delonix regia, and it is known for its vibrant red or orange flowers. Gulmohar trees are commonly found in tropical and subtropical regions.
The wheat plant is a grass, not a shrub. It is not an herb because an herb is a plant that is valued for flavor, scent, medicinal or other qualities that wheat does not have.
grass
The Esperanto words for grass and herb are herbo and herbojn.
Sugar cane is neither an herb nor a shrub. It is a grass.
Rice is a monocotyledonous grass - it it neither a shrub nor herb
Most likely a shrub, because most herbs are used for medical reasons.
'une herbe' (fem.) is a French word for herb, weed, and grass.
marijuana is classified as a woody grass.
it's a herb because there are no woody parts at all
grass, herb
Sweet grass
Neither. Barley is a grass that is used as a cereal crop.
Lemongrass