"humans ingest proteins to build muscle and connective tissues and to form enzymes and antibodies. why do we consume carbohydrates? "

In humans, proteins form the structural as well as plays the functional role like as a catalyst but a human diet requires 50-55% carbohydrate.

This accounts for the reason as the carbohydrates form like glucose is oxidized or broken down which provides energy to perform various functions.

The human metabolism is programmed in a way that it first utilizes glucose as a reactant instead of other sources like proteins in cellular respiration to form ATP molecules.

This shows that carbohydrate is the primary source of energy to the human and thus consumed by humans in the large amount.