Blood in urine can be a sign of kidney damage. why is blood not usually found in the urine of a healthy person?

Blood is not present in the urine of healthy person since the blood cells, platlets and plasma proteins are large enough to prevent their glomerular filtration. These components of blood are not present in the glomerular filtrate and hence, are absent in urine as well.

Explanation:

The first step in urine formation is glomerular filtration which filters the nitrogenous wastes and other compounds and forms glomerular filtrate. Glomerular filtrate undergoes tubular reabsorption and tubular secretion to form urine.  

The components of blood such as blood cells, platelets, proteins of plasma are not added to the glomerular filtrate. They exit the glomerulus of nephrons via efferent arteriole. Since blood components are not filtered to become part of glomerular filtrate, blood is absent in the urine of a healthy person.