Because what you think (and what they think) is healthy is not. It's that simple.
First of all, no, it's not simple physics. Your body adjusts its basal metabolic rate based on the caloric imbalance to which it has access. In other words, you have to consistently overeat by a heck of a lot in order to put on weight. Similarly, you need to undereat in a similar fashion to lose weight. When I talk about overeating and undereating, keep in mind that I'm talking about in relation to your total caloric requirements, which includes exercise.
Access is the important issue here, and it's why people say that weight gain is a hormonal thing. Yes, but the hormones are only related to genetics, not pre-determined by them. The major hormones in question are insulin and cortisol. If you're provoking an insulin response with your diet and your body isn't clearing the insulin quickly enough, not only will you gain fat, but you could lose muscle as your muscles become desperate for energy and cannot access your fat stores. Cortisol is a bit different, but certain exercises that are deemed "healthy" by common knowledge release cortisol into your blood. Cortisol tells your body to put away fat in your belly, prioritizing muscle burning. This is the primary cause of what's known as being "skinny fat," more clinically as normal weight obesity.
Keep in mind that these hormones are controlled somewhat by genetics, and that cortisol can also come about from general stress. Really, though, the issue of genetics is when you're eating a diet that's unhealthy (in spite of claims otherwise) or doing exercises that are unhealthy (again, in spite of claims otherwise).
So, basically, those people that you know are eating things that are unhealthy that they're told are healthy, avoiding healthy things that they're told are unhealthy, and doing exercises that aren't good for them that they think will give them longevity.
Edit: A couple of notes, I suppose: I weighed over 400 lbs just a year ago. I've been eating healthy (haven't even been exercising that much, to be honest) and, since then, I've lost about 100 lbs, which means that I'm still over 300 lbs. I've lost weight, but eating healthy doesn't mean losing the weight instantly. That would, in fact, be unhealthy. It's not really about whether or not they have weight, it's more about whether or not they're losing weight at a healthy pace (not too fast).
The other note would be that it is true that different people have different body types, to the point that our modern society views them as overweight. This is not necessarily the case, especially if you're just looking at the scale. If I suddenly dropped down to a healhty BFP, I'd still be considered obese because of how much muscle mass I have. Some people's body types are thicker (not necessarily fatter, and while ideal BFP varies, it's usually within a small normal distribution) than others. Furthermore, healthy BFP goes up as we age, so someone that's 40 and has a higher BFP than a 20 year old that's considered healthy is actually healthier than a 40 year old with the same BFP as the 20 year old.