You're not wrong about the root cause of chronic medical conditions. Depending on how you count, something like 1/7 of all healthcare spending now goes to type-2 diabetes and related conditions, and nearly 100% of those cases are caused by lifestyle issues.
But it's unfair to blame doctors. In a typical office visit they only have a few minutes with the patient which isn't enough time for useful lifestyle counseling. And it isn't even really their job anyway; diet counseling should be provided by Registered Dieticians who are specifically trained in that field. Any real improvements will require major national political policy changes to better align incentives and shift resources away from treatment and towards prevention.
But it's unfair to blame doctors. In a typical office visit they only have a few minutes with the patient which isn't enough time for useful lifestyle counseling. And it isn't even really their job anyway; diet counseling should be provided by Registered Dieticians who are specifically trained in that field. Any real improvements will require major national political policy changes to better align incentives and shift resources away from treatment and towards prevention.