Pretty much any teacher will tell you it's because parents aren't involved and treat school like daycare. When Mom and dad come home and veg out on their phones and TV after work, the kids learn that as well. The ones that succeed have parents that are involved with their schooling which is less and less likely these days. Teachers are quitting left and right because of it.
Lol kids are in school 30+ hours a week. When you look at the basic curricula they have to learn it is 99% on the school if it cannot be taught in that time frame, even if the kid is going home to a 4 hour shift in the slave labor camps and only ghouls for parents.
There are many reasonable criticisms of modern public schooling, but the claim that's it's completely broken (and needs burnt to the ground, as many conservatives will claim) is hyperbole and unhelpful to making actual productive change.
And the notion that private schooling (in general) is better is hard to believe. When we looked at private schools for our son, test scores and college admittance were only marginally higher and much of the gap was simple selection bias (private schools are not legally required to take all students, so don't deal with disabled, disadvantaged, or otherwise non-exceptional students). The only time private was substantially better was hyper-elite, hyper-expensive schools (Sidwell Friends, DC vs Paul VI, Fairfax vs the publics in FCPS).
I did checked them. And while it is not perfect, it is not a massive horrible disaster conservatives like to make it. It is just not unfixably broken as it is called. Compared to Europe, it does fine, being above average or around there depending on the test.
Also, America do tend to be country of extremes, so it has some very good public schools and some bad ones. Bad ones being in poor place. And it just so happen that the countries doing better tend to have less poverty and less issues related to it.