If horses can digest grass with only one stomach why do cattle need more than one stomach?

Both are herbivores, but one is more efficient at digesting forage (a.k.a., grass) than the other. Cows tend to be more efficient at digesting forage because of their four-chambered stomachs that enable a more thorough process of digestion of roughage, and also enables a more wider range of forages that these ruminants can eat.

Horses, on the other hand, are not as efficient, nor are as good at digesting a wide range of forages, including higher-nutrient-level forages like legumes. But going back to the comparison of digestive anatomy, horses are less efficient because their stomachs are built like ours, which cannot digest roughage as efficient as ruminants can, and instead depend on their cecums to break down partly digested roughage through the process of fermentation. But these hind-gut fermentors do not have the same method of fermentation as cows do, and thus the digesta that cannot be digested anymore has to be expelled.