America doesn't have as much social class mobility as we might think. People who do well now generally have kids who do pretty well. There are a lot of reasons for this, but one that's kind of obscure is the increasing complexity of the meritocratic ladder, often justified in the name of Rights. The result is that kids with smart, hard-working, married parents can get an edge on the system in multiple ways.
For example, when I was a kid, taking the SAT was pretty simple. For example, you didn't have any right to look at your test once you turned it in. But, now, you have the "right" to order (for a non-nominal although not extortionate sum) from College Board your SAT test booklet along with a reproduction of your answer sheet (at least for certain test dates) so that you can study what you messed up on. I just spent two hours looking at my son's May test performance looking for patterns in his mistakes so he can do better on his next try. It's an intellectually challenging process.
I suspect there are a lot of little angles like that that have emerged in recent decades that help upper middle class families stay ahead of broken lower middle class families.