Why can't doctors cure anything?

Why is it that if I had any disease like cancer or blindness or psoriasis, the doctor cant do anything to make it go away? If I had a severe injury or infection, why could I still die even if Im already in a hospital? The hospital has all these computers, and machines looking at your body and medicine, and all this stuff, yet so many people still die in hospitals. If I had the worst cancer, why cant doctors fix it?