The complexity of digestive apparatus found in Bovidae family and other ruminant mammals is just an optimization. Herbivore monogastric mammals like horses consume cellulose too. There even isn't that necessary for the mammal to be (specialized) herbivore in order have the cellulose digesting ability, as pigs clearly demonstrate. Some extensions in our menu limits are always welcomed. Digesting lactose as adults was nice, but digesting cellulose, even if not that pleasant or efficient, would have been even nicer, as when things went historically bad, it would had came in handy if we could literally just run to the forest for more than to just hide.
An enzyme to break down milk seems to be such an easier adaptation compared to a whole series of stomach, which are basically a series of fermenters.
Having raised goats the size of their rumens to body is pretty amazing.
Or strong acids and 190°C over a catalyst.
Cellulose is remarkably strong, especially with lignin in the structure too. We're lucky anything can digest it at all.
Which brings me to mycology, probably the best way to extract energy from cellulose for humans. Sterilize, inoculate, keep moist, and harvest a significant percentage of the source material as good carbohydrates and protein.
Which is digestion of course, but it isn't that cows can do it. Our large intestine does the same thing but it isn't anywhere near big enough to get a significant amount of energy from cellulose.
My guess: grazing takes extremely long and so we let cows do the boring job of grazing outside all day then we quickly slurp up whe harvested calories in just a few glasses of milk.