Funny thing about my sister in law, she eats very healthy but has high cholesterol, my wife eats like I do which is to say "American " and has no health issues at all.
That scenario is only one of billions.
For instance, I know someone who used to drive drunk when they were younger and they were never in a crash. That doesn't mean it doesn't matter whether we all drive drunk or not. Sure, a car crash can happen at any moment, even while sober, but it's about decreasing the odds of it. It's factual that driving sober decreases the odds of being in a crash, as it is factual that eating healthy increases the odds of living a long, healthy life.
Not everyone who eats McDonalds on a daily basis is going to have serious health issues or die of a heart attack at 55, but doing so will greatly increase the odds of that. Not every healthy eater is going to live a long and healthy life, but again, it's not about guarantee, it's about increasing the odds, and in that sense, it
does matter how you eat.
In the end, I don't think anyone here, including myself, minds that you eat everything you want to eat, and you shouldn't mind that others want to eat healthy.