Why is there a large amount of blood capillaries in lungs digestive system and muscles?

There are many capillaries in your lungs, digestive system and muscles because each of these locations require blood to be transfered to individual cells snd capillaries are the smallest vessels that can do this.
In your lungs capillaries connect to the alveoli and enable gas exchange within the body.
In your digestive system capillaries enables food to be passed through the walls of the intestines.
In your muscles capillaries deliver a fresh supply of oxygen to each cell and remove any bi-products enabling you to move sufficiently.