We aren't discussing individual teachers and the hard work they put in to students. We are discussing the U.S. education system. The lack of parental support as well as the issues with society that you discuss contribute to the problem with the system that we have. They are not separate from each other.
http://www.theatlantic.com/education/archive/2016/05/detroits-school-crisis/482010/
I highly recommend this article from The Atlantic, which discusses the disaster that is Detroit's school system. Granted, this is an extreme example, but this is what can happen in our country.
I think people forget that just because they are in a good district with teachers, students, and parents who care as well as have the necessary funding, does not mean that there are districts out there who do not. Unfortunately, school districts can be winners and losers for many reasons. I played tennis in high school, and we were fortunate enough to have nice hard courts to play on. When we went to other schools to play, I remember one specifically: the students had to play on asphalt concrete courts (as in the same stuff they use in the streets) that were in terrible shape- cracks everywhere, so not exactly safe. Yeah, it's "just tennis", but how in the world is that fair? What does that say about us all having an "equal starting line" overall? The fact of the matter is, bad parenting aside (because that can definitely be a contributor), it is not equal. Some districts and the kids in them get a raw deal due to an education system that does not live up to expectations.