Yeah, and that's mostly true and all, but at the same time, if your family and friends ought to be building up your habits, they can't do it when you're somewhere else. If your child is often away from you and you know you're a person with the good kind of knowledge (because being "clever" doesn't mean you're not a bad person), or they were somehow with people who weren't raised properly, they'll learn not-so-good habits. Fast forward 10 years, and you've got a bad person. Look to all the people in the world, and you've got diversity.
We know what we ought to do, but sometimes it's pretty hard to overcome your strong impulses or inclinations to do something base. Maybe you've got this craving for glazed donuts and you eat four every day. Then you go to McDonalds because you keep passing by the place when you're walking to save gas, and every single time, you happen to be hungry. And then maybe you go home and you're itchin' to play that awesome video game you just recently got instead of doing homework when you know (read: think) you ought to. Each time, your reason lost to your inclinations, and you're labeled a "morally weak" person by Aristotle. Something Aristotle never says is how we can overcome this life where we are trying to hit a mark and continually missing until we get close. He just basically says "weeeeell, if you're fortunate, and you have friends, and you know how to exercise your wisdom, and eeeeeverything falls into place, then you've got a good life, but if you continually screw up when you're young and no one sets you straight, you're too deep into it, bro. You only got one chance at life, and life is HARD. If you aren't lucky as well as all of the above, sorry Charlie."
Normal people (i.e. not diseased, badly habituated, innately corrupt, or otherwise brutish) get that in general, killing is bad and respecting others is good and so on and so forth, based on the fundamental laws of nature, but when they've been poorly raised, or just not raised at all, they're at a loss. No idea how to proceed, and literally guessing what's better or worse for them, letting fate give them their life and having it turn out however it's going to turn out. When you don't have enough of an education, you tend to turn to conformity, and that's the danger. Conformers don't think so much as just doing something. Knowing how to prevent obesity is learned information, and if you've never seen a chubby person, you won't know to not-eat in a similar way or not-exercise in a similar way. So in a nutshell, obesity is a problem because those people weren't fortunate enough to be taught or learned in a way that steered them away from that lifestyle, and well... that's how it turned out for them, and they just have a hard life ahead (unless someone helps, of course).