Child goes home, does homework, doesn't understand something. Child asks parent. Parent helps child understand.
Even more with reading, most parents flat out don't read to their kids anymore, and don't help them with their reading assignments. So you end up with a worst case scenario where kids can't fucking read and can't spell either because they communicate with each other through slang while texting online.
An education system where parents are disengaged is an education system doomed to failure. Teachers do not have enough time in the day to adequately fill the gaps.