Because big annuals need to be replanted every year. Thus the name - annual. The big advantage in grass pasture is it is perennial - which means it doesn't need to be replanted. The big advantage for farmers and ranchers is feeding cattle better feed at much lower cost. That comes from not having planting costs. And doubly so if you graze it and have no harvest cost too ! An added benefit is perennials are way better for the environment.