I was mainly thinking about soybeans, not humans eating grass. 75% of soy bean production goes to animal feed, and more than 50% of US, and 40% of worldwide, grain production goes to animals instead of directly to humans, where it's inefficiently converted to energy humans consume. All this because people like the taste of meat and because of tradition.
Other resources, such as fossil fuels and water, are used at a much higher rate when raising animals than when producing crops. Grain-fed beef production uses 100 000 liters of water per kilogram of food produced, broiler chicken 3500 liters per kilogram, and soybeans 2000 liters per kilogram. Water is a precious resource in many places of the world.
This is the way the world works today, but you don't argue for animal products over vegetables, you argue for grass-feed animals instead of crop-fed. You should vote with your wallet and only buy grass-fed meat, both when you cook at home, when you buy an animal-based snack, or when you go to a restaurant. I think that many meals would have to be vegetarian, because a purely grass-fed meat, dairy, and egg diet would be hard to uphold.