Doctors, with their few minutes per patient, are not in the position to teach or really influence people's habits. Teachers should teach- about exercise among other things.
In some countries they actually do give such referrals. But it's rare in the US because most insurance plans don't cover such treatments. That is starting to change to a limited extent; my retired father receives a discount on his medical insurance for going to the gym every week.
Also, parents should parent. And people should take care of themselves. I'm not saying doctors should start teaching anything. I am definitely saying that they should NOT be prescribing statins flip-ly when getting someone to diet and exercise is the correct answer.
Medical training does not cover nutrition in any depth, I'd say your average doctor is under-qualified in addition to time poor.