In absolute, changing diet is not inherently hard : it's pretty easy to eat a balanced diet and get pleasure from it.
But it's not enough to just say "eat better", "do more sports" : people can't change the way the lived their whole life if you don't educate and help them to make the change. The education on this topic is so poor that we are still confusing balanced diet and low-calorie diet.
And then there is the elephant in the room which is the refined sugar : for some dubious reason, most scientists agree that refined sugar acts like a hard drug but nobody officially wants to call it a drug. Recent research are saying that the addictive effects of refined sugar are akin to heroin. That's a serious issue then. What it means is that a doctor asking his patient to eat less sugar / eat a balanced diet without further guidance or help is just like saying to an heroin addict that he should stop.
I'm relatively overweight and pretty self educated on the topic but I still decided to work with a nutritionist to change my diet and we are talking about months if not years of follow-up. Eating habits are largely automated by our biology : willpower and education are nothing against an hypoglycemia induced sugar craving.
Like drugs, most people need help and guidance on the long term to change those habits. But what's even worse is that unlike drugs, unhealthy products are basically everywhere in your supermarket, your TV ads, your billboards, your friends lifestyle, and even worse, profoundly ingrained in the culture. You have to fight the "drug" but also the whole capitalist world around you.