It might be how metabolism works, but clearly it's not how human psychology works.
In the same way that some people drink massive amounts of alcohol in college and turn into alcoholics ruining their lives and liver, and others just 'phase' into a more healthy lifestyle, clearly some people find it harder to do what you did than you.
One conclusion is that they just don't try hard enough, and they're not as 'good' as those who do. They willingly choose to live a lesser life. Because they're lazy or whatever.
The other, more complicated (and I'd argue less arrogant and more empathic) conclusion is that the psychology of addiction is different for different people, and approaches that work for one person don't work for others. And rather than saying 'it's really that simple', we should research the problem and devise solutions that take into account the simple fact that some people apparently drink until their liver gives up and all that's good in their life is eventually gone, or that some people apparently eat and grow fat and hate themselves and live 'lesser' lives instead of getting in shape.
Having spent a lot of time with 'highly successful and driven' people, and having spent a lot of time with fuck-ups who suffered from a variety of addictions, I've come to the conclusion that being that hard to define 'good' has little to do with it. Instead, it seems that the difference is mostly upbringing and environment (where this environment was usually not a choice, but rather a circumstance).
I've been lucky to be surrounded by both happy, successful people and major 'fuck-ups', and as a result my life currently occupies a mostly happy space. But I have no illusions that this is primarily the result of my personal choices and willpower, as willpower itself is a result of upbringing and environment.
Sure, my will and choices matter. I don't want to remove personal agency entirely. But even if my current (happy) state of being is 50% a result of factors outside of myself, it's enough to realize that I'm fucking lucky, and those who fare worse than me need support and researched solutions rather than 'it worked for me, so it should work for you'.