It's all of it together. It's easier for parents to just stock their pantry with cheapo snacks that the kids can raid after school than for them to provide healthy alternatives and expect the kids to eat them. It's easier to hit McDonald's three times a week and eat crappy, high-fat meals because mom or dad is too wiped out from working to cook a decent dinner most of the time. If there's both a mom and a dad, it often doesn't make any difference because they're both working anyway and it just means that they can afford to dine out every night, not just sometimes. Meanwhile, kids just sit on their asses playing games, or watching TV, or on the Internet. Even socializing no longer requires walking or riding your bike over to someone's house; you just Skype and/or hang out with them inside a virtual world.
Parents used to literally chase the kids out of the house, not only so that they (the parents) could have some peace and quiet, but because it's good for the kids to actually get some fresh air and exercise. Even if all they did was walk around bored, or head to the mall and be bums, they were at least walking, doing something physical. When no one forces them, most kids today would just sit on their asses getting fat, just like their parents.