1. Diet and exercise is secondary to drugs? I mean, sure, if there is an accute condition. You can't bench press your way to beating bronchitis, for instance. But taking care of yourself will keep your body working better, which might help lower the chances of illness with a healthy immune system.
2. Medical doctors are trained in the human body. They know how the body is supposed to work, and can try to isolate and eliminate the things that cause the body harm. Some specialize less about general wellness and more into specific ailments and their cures, but you only see them if there is a problem. Similarly, a mechanic usually has the know how to keep a car properly maintained, but often specializes in fixing problems/repairing something that is broken.
3. Screening doesn't "prevent" illness, sure. It just helps catch things early and monitors things to try to eliminate problems before they get severe.
4. The US Healthcare system is busted. It also doesn't "prevent" illness. In theory, it is supposed to help make sure that the person can help stay healthy and cover expenses any treatment would cost. By generally helping people keep costs down it is supposed to encourage a generally healthier population.
5. Disease isn't necessarily genetic. Sure there are genetic issues (higher chances for high blood pressure, for example) or otuer inherited problems, but that's only part of it. Ebola isn't "the genetic hand you've been dealt." People can be more vulnerable to heart disease because of genetics, but it isn't the "cause" with few exceptions.
6. More research is always needed. Can always do things better, illnesses change, and breakthroughs happen. Saying cures won't come from the medical field, what? Will sugar pills, prayer, and crystals find the answers?
7. Depends on what for. If it's an emergency or particularly direct, hospitals are good, but for general medical care you don't need to. But if say, someone gets shot, has a car accident, or has a stroke where you you suggest they go instead?
8. Depends on the illness. Chicken pox, for instance, if you get it young you don't need a vaccine, but it gets riskier the older you get. Some peoplws will be fine without a vaccine, others not. It can affect the body differently. It's part of the reason research is helpful.