I suppose this could be a terrible question because it's so obvious as to why. However, I just don't see it I guess. When you look at animal animal on pasture: Cattle, goats, horses, sheep, chickens ect ect ect we always have such a high percentage of grass in the pasture. Some pastures are even solely grass. Much more common is grass mixed with clover, alfalfa or another legume for it's added nutritional benefits depending on the animal grazing the pasture. You can go a step further and include a higher amount of legumes with diversities in species along with other "medicinals" like plantain, chicory, dandelion, kale, comfrey, chives ect and even go as far as including hints of the Lamiaceae family ie mint, basil, sage, rosemary.
But why is grass typically the staple? Why isn't chicory a staple? More nutritious than pasture grasses and similar DM yeilds/acre. Why not throw in a much higher percentage of the "medicinals" to fulfill more of an animals diet in place of grasses?
I do understand that in conventional agriculture grass is most likely used due the fact that it takes over everything else making it very easy to maintain your crop without re-seeding along with the combination of dumping N on a field for high yields.
Down sides to chicory, for example sake, would be that it's high nutritional value and 90% digestibility rapidly declines when the plants goes beyond the vegetative state into a bolting state. This has proven hard to keep chicory as a quality forage if it grows too fast- "Forage chicory can grow rapidly during the months of July and August when temperatures can exceed 90 degrees F".
So, what are your takes on it? Please, keep in mind I am not attempting to make this about chicory>grass for a pasture staple. Any other species could have their case argued; I just picked one of many for examples sake.. The main question at hand is what makes grass a more viable option than all the others?
If interested in chicory as a forage here is the article I was referring to throughout.
https://ohioline.osu.edu/factsheet/VME-31