I know people have said stuff like "not all" and can be "related to puberty" but I still resent the accusation (which is what I would call it), that acne is caused by diet.
From my own observations at least, I have seen no association between diet and acne (where acne is simplified to an on/off thing). I'm not talking just one or two friends whose diet is terrible but don't have acne, or visa versa - but no association across the board whatsoever.
I'd say many people not only eat appalingly but drink like fishes too, and do they all have noticeable acne? No, a minority of them have acne (sort of you know, as you'd expect at random).
How many people who eat well have acne? Still a minority, but no smaller proportion than those who have an extremely poor diet.
I know one poor kid who throughout secondary school/college had the most awful disfiguring acne, which was eventually thwarted only with roaccutane - his family eat a traditional japanese diet. You know, the one that's meant to be so much healthier than ours? Vegetables and fish and soy-based products.
I've had mild/moderate acne throughout my teens and not only do I eat pretty well (honest to god I eat better than most), it's more than that - even when going through periods of poorer eating, poorer lifestyle, I have noticed no change in my skin.
The only things which have affected it's baseline level (eg: can be controlled a little better with careful washing/ degreasing) are the passage of time and hormonal contraceptives.
My brother has never suffered with any degree of acne, and his idea of a healthy meal is baked beans with his jacket potato and cheese.
Maybe some people can improve their acne with dietary changes, who am I to say they haven't - but I don't think its fair to imply diet as a cause for acne. (Maybe noone here really has tbf, I'm not singling anyone here out, it just seems to be a popular opinion). Some people are just prone to it, and others will never get it.
Admitted, the research isn't top quality (not RCTs), but the reason your doctor won't tell you acne is caused or controllable by diet is because it doesn't strongly seem to be the case, as going by the evidence. If it is a factor, it's no where near as important as other non-controllable factors, pretty much related to luck of the draw, puberty and whether or not you're taking roaccutance or smearing antibiotics all over your face - evidence based treatments.
I've run out of puff and really not wrapped this up at all but I can't be bothered so bla