Doctors typically have to consider how likely a patient will be to follow through with their instructions. A recommendation of popping a fairly cheap pill each morning is more likely to be followed than 30 minutes of sun. This is the same reason doctors hand out pills for high blood pressure instead of insisting that patients lose weight and cut down on Sodium. It's sad that they have to rely on pills for everything, but human nature being what it is, the pills tend to be more effective.