My assumption is basically that they live like that and raise their children like that because a) they don't know better (about nutrition/eating properly), b) they don't care/don't have the energy to tackle the problem. Some combination of the above.
I'm sure they had a lot of other things going on in their lives too. As someone who doesn't have kids, I feel like the timesuck of having multiple children and raising them "well" in all the other vectors a person might possibly want to raise their children well in - quite reasonably reduces the amount of time and energy a parent can direct towards self-improvement. Or changing what's always worked for them. Or teaching their children an entirely different lifestyle from what the parents know and live. It'd require a re-self-education and then application and re-teaching of that new knowledge to 1, 2, however many children. And even if you can change the lifestyle, the kids already absorbed what life was like before the change for however long that was.
I just felt really bad for the younger kid. who was obese at like 10.
Sure, sometimes people seem to raise perfect kids, but for the most part I think it's impossible. You can raise your kids as best as you can and that's about it. Even just a blind spot in the 'wrong' vector -- any possibly significant vector, which is like almost all of them -- can leave a kid vulnerable or disadvantaged. Guess what! That's being human for ya.
I don't know any adults who are perfect (in my life, anyway). Why expect parents to produce children who excel in every way and are in control of and smart about all their choices, when the parents aren't like that and also no one in society really expects that of them?