Some are. But unless they are in a truly rural area, that is just not true. According to recent numbers 80% of people in America live in urban areas.
Those urban areas, while often not designed for bicycles are a lot more frindly to bicycles then true rural areas. Saying kids are trapped untill they have a car wrong most of the time. Yeah they may have to bike or walk a few miles to get anywhere but that's not a big deal.
The people living in true rural America truly do not have the ability to do so. But again, that's less then a quarter of the populations and likely far less then a quarter of all kids.
We have a culture that prioritizes physical comfort above most other things. A car is more comfortable so that is what most people what to do.