Because you would die..
You need proteins to live
I knew a 30 year old vegan who died of starvation. Eat decent food.
Your muscles would shrink
Muscles would atrophy. We need protein to keep muscles healthy. If you don't get enough protein in your diet your muscles would cannibalize themselves for the protein.
For the lazy: "Your body uses proteins in your diet to build new cells, maintain tissues, and synthesize new proteins that make it possible for you to perform basic bodily functions."
"About half the dietary protein that you consume each day goes into making enzymes, which are specialized worker proteins that do specific jobs, such as digest food and assemble or divide molecules to make new cells and chemical substances. To perform these functions, enzymes often need specific vitamins and minerals.
Your ability to see, think, hear, and move — in fact, to do just about everything that you consider part of a healthy life — requires your nerve cells to send messages back and forth to each other and to other specialized kinds of cells, such as muscle cells. Sending these messages requires chemicals called neurotransmitters. Making neurotransmitters requires — guess what — proteins.
Finally, proteins play an important part in the creation of every new cell and every new individual. Your chromosomes consist of nucleoproteins, which are substances made of amino acids and nucleic acids."