Once you realise the incredible power of nutrient recycling in domestic livestock you become aware of what a significant evolutionary advantage it afforded Palaeolithic people.
For me to get the equivalent daily calorific payload from plants I would have to clear acres of land.
It's really interesting how hard it is to get calories. The Europeans who landed in North America eventually combined Northern European, North American, and Meso and South American agriculture to maximize calories during the mini-Ice Age: corn, potatoes, sweet potatoes, beans, animal husbandry/dairy. GRains simply take a ton of land per calorie.
To your point, check out how many calories this guy gets per day with a few sheep:
Dairy and lactose tolerance allowed the Mongols to travel on their own supply lines - they drank mare's milk.