Why do doctors give glucose rather than starch as intravenous injections?

Think about it logically. You just answered your own question. You said the starch didn't go through the membrane, but the glucose did. The starch molecules are much larger than the glucose molecules, and would take significantly longer to diffuse through the membrane if at all. The same would work inside the body. Glucose can enter the cells through the PM much easier than starch can.