They're doctors, not CEOs. They are advising based on the behaviors they see from most of their patients, who probably come in asking for quick solutions and are unable to make lifestyle changes stick. Patients who are educated about their own conditions, willing to listen to advice, and able to keep to that advice over the long haul are a very small proportion. Not that doctors shouldn't offer this sort of advice anyway, I am just asking you to please try to understand why they behave in such a way.
I was talking to my physical therapist this morning about my experience with the recent exercises he'd given me, and I pulled up my Garmin workout calendar to show him my inconsistency. He'd told me to do a particular stretch every 3 hours or 6x/day, and I'd been having several days a week where I'd only completed the routine 4 or 5 times.
He said that level of consistency was fantastic, that at least a third of his patients flat-out told him they hadn't done any of the exercises at all, another third showed no improvements above baseline and he suspected they had lied about it, and the remainder had moderate compliance. When he'd told me 6x/day, he was anticipating 2x on the high end. We adjusted to 4x/day, where morning, lunch break, after work, and before bed were easier habits to stick to than trying to drop and do press-ups in the middle of my 9:00 meetings.
And that's at a sports and fitness-focused PT organization, not an average general practitioner working with median diets and advising a society that by default trends towards diabetes.
