Medicine doesn't really ignore nutrition, but the problem is:

1. Most people don't believe it anyway. People want to hear they can eat hamburgers and milkshakes and be healthy. Telling them "we know that gives you heart disease and cancer" does nothing.

2. Nutrition is complicated and different for every person, because everyone has different things they can tolerate. The "perfect" diet is actually worthless because it has a 0% success rate. Really, we have to optimize for how miserable people are willing to be.

3. Most people are unhealthy enough that nutrition is the least of their concerns. That sounds crazy, I know, but if you're obese (which most people are!), then priority is being not obese. Not your nutrition. I know those sound related but they're way less related than you think.