Corn, wheat, sugar, rice and other sources of carbohydrates are grown at low cost. The volume of protein from legumes and animals that can be produced from the same input of resources is much smaller.

Carbohydrates are plants’ mechanism for storing energy.

Starch is chemically stable over the long term.

So are proteins, actually, but most protein-rich foods are high in fat, and fat is not chemically stable. In the presence of oxygen, fats go rancid.

So, you can put grains someplace dry, and eat them decades, centuries, or even millennia later. (Not hyperbole – we’ve taken wheat grains out of Egyptian tombs and planted them, and they sprouted.)

Carbs come from plants; proteins come from animals. The amount of corn, rice, wheat, barley, etc (all grains) and the yield per acre of land is FAR GREATER than the yield of meat from cattle, sheep, etc per acre of land… and the resources to promote growth, health, and harvesting of the respective “crops” is far less for plants than for animals. It costs a LOT more to produce a pound of burgers than a pound of corn. And of course, many pounds of corn are needed to feed the cattle that you get the pound of burger from.