Why isn't whole blood given to a patient with extreme blood loss instead of packed rbc?

Good question. Whole blood includes different components-- platelets, red blood cells, and clotting factors. For most patients, just replacing the red blood cells fixes the acute problem. Then the other portions of the blood can help other patients. When you donate a unit of blood, it is separated and can help up to a total of 4 other people. In some rare situations, it is appropriate to use whole blood.
Great question. Excellent question as in combat it is used, although not as much as prbc's. In general it is not used in the civilian world for fear of higher transmission rates of things such as HIV and hepatitis. Because of the success of whole blood in combat situations the ratio for componet blood therapy has been changed to 1:1:1 to better simulate whole blood.